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Understanding message size limits

 

Applies to: Exchange Server 2016

Topic Last Modified: 2016-03-28

Learn how administrators can apply limits to messages in an Exchange 2016 organization.

Administrators can apply limits to messages that move through their Microsoft Exchange organization. You can restrict the total size of a message or the size of the individual parts of a message, such as the message header, the message attachments, and the number of recipients. You can apply limits globally for the whole Exchange organization, or specifically to a connector or user object.

As you plan the message size limits for your Exchange organization, consider the following questions:

  • What size limits should I impose on all incoming messages?

  • What size limits should I impose on all outgoing messages?

  • What is the mailbox quota for my organization, and how do the message size limits that I have chosen relate to the mailbox quota size?

  • Are there users in my organization who need to send or receive messages that are larger than the maximum allowed size?

  • Does my organization include other messaging systems or separate business units that require different message size limits?

This topic provides guidance to help you answer these questions and to apply the appropriate message size limits in the appropriate locations.

Contents

Types of message size limits

Scope of limits

Organizational limits

Connector limits

Server limits

User limits

Order of precedence and placement of message size limits

Messages exempt from size limits

The following list describes the basic types of message size limits, and the message components that they apply to.

  • Whole message size limits   Specifies the maximum size of a message, which includes the message header, the message body, and any attachments. You can configure message size limits on incoming messages and outgoing messages. Exchange uses the custom X-MS-Exchange-Organization-OriginalSize: message header to record the original size of the message as it enters the Exchange organization. Whenever the message size is checked, the lower value of the current message size or the original message size header is used. The size of the message can change because of content conversion, encoding, and transport agent processing.

    noteNote:
    For any message size limit, you need to set a value that's larger than the actual size you want enforced. This accounts for the Base64 encoding of attachments and other binary data. Base64 encoding increases the size of the message by approximately 33%, so the value you specify should be approximately 33% larger than the actual message size you want enforced. For example, if you specify a maximum message size value of 64 MB, you can expect a realistic maximum message size of approximately 48 MB.
  • Attachment size limits   Specifies the maximum size of a single attachment in a message. The message might contain many smaller attachments that greatly increase its overall size. However, the attachment size limit applies only to the size of an individual attachment.

  • Recipient limits   Specifies the total number of recipients that are allowed in a message. This includes the total number of recipients in the To:, Cc:, and Bcc: fields. A distribution group counts as a single recipient.

  • Message header size limits   Specifies the maximum size of all message header fields that are present in a message. The size of the message body or attachments isn’t considered. Because the header fields are plain text, the size of the header is determined by the number of characters in each header field and by the total number of header fields. Each text character consumes 1 byte.

    noteNote:
    Some third-party firewalls or proxy servers apply their own message header size limits. These devices may have difficulty processing messages that contain attachment file names that are greater than 50 characters or attachment file names that contain non-US-ASCII characters.

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The following list describes where message size limits are applied in the Exchange organization.

  • Organizational limits   These limits apply to all Exchange 2016 servers, Exchange 2013 Mailbox servers, and Exchange 2010 Hub Transport servers that exist in your organization. On Edge Transport servers, any organizational limits that you configure are applied to the local server.

  • Connector limits   These limits apply to any messages that use the specified Send connector, Receive connector, Delivery Agent connector, or Foreign connector for message delivery. Send connectors are defined in the Transport service on Mailbox servers and on Edge Transport servers. Receive connectors are defined in the Transport and Front End Transport services on Mailbox servers, and in the Transport service on Edge Transport servers.

    You can assign specific message size limits to the Active Directory site links in your organization. The Transport service on Mailbox servers uses Active Directory sites, and the costs that are assigned to the Active Directory IP site links as one of the factors to determine the least-cost routing path between Exchange servers in the organization.

    You can assign specific message size limits to the Delivery Agent connectors and Foreign connectors that are used to send non-SMTP messages in your organization.

  • Server limits   These limits apply to specific Mailbox servers or Edge Transport servers. You can set these message size limits independently on each Mailbox server or Edge Transport server.

  • User limits   These limits apply to a specific user object, such as a mailbox, mail contact, mail user, distribution group, or a mail-enabled public folder.

The following tables show the message limits, including information about how to configure the limits in the Exchange Management Shell or the Exchange admin center (EAC). To learn how to open the Exchange Management Shell in your on-premises Exchange organization, see Open the Exchange Management Shell.

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Size limitDefault valueExchange Management Shell configurationEAC configuration

Maximum size of a message received

10 MB

Cmdlet: Set-TransportConfig

Parameter: MaxReceiveSize

Mail flow > Receive connectors > More options More Options icon > Organization transport settings > Limits tab > Maximum receive message size (MB)

noteNote:
In the EAC, you can only set the values 10 MB, 25 MB, 100 MB, or unlimited.

Maximum size of a message sent

10 MB

Cmdlet: Set-TransportConfig

Parameter: MaxSendSize

Mail flow > Receive connectors > More options More Options icon > Organization transport settings > Limits > Maximum send message size (MB)

noteNote:
In the EAC, you can only set the values 10 MB, 25 MB, 100 MB, or unlimited.

Maximum number of recipients in a message

500

Cmdlet: Set-TransportConfig

Parameter: MaxRecipientEnvelopeLimit

Mail flow > Receive connectors > More options More Options icon > Organization transport settings > Limits Maximum number of recipients

noteNote:
In the EAC, you can only set the values 100, 1000, 5000, or unlimited.

Maximum attachment size for a message that matches the conditions of the Transport rule

Not configured

Cmdlets: New-TransportRule, Set-TransportRule

Parameter: AttachmentSizeOver

Mail flow > Rules > Add Add icon > Create a new rule, or select an existing rule, and then click Edit Edit icon.

Click More options.

Use the condition Apply this rule if > The message > size is greater than or equal to, and enter a value in kilobytes (KB).

Maximum message size for a message that matches the conditions of the Transport rule

Not configured

Cmdlets: New-TransportRule, Set-TransportRule

Parameter: MessageSizeOver

Mail flow > Rules > Add Add icon > Create a new rule, or select an existing rule, and then click Edit Edit icon.

Click More options.

Use the condition Apply this rule if > The message > size is greater than or equal to, and enter a value in kilobytes (KB).

To see the values of these organizational limits, run the following commands in the Exchange Management Shell:

Get-TransportConfig | Format-List MaxReceiveSize,MaxSendSize,MaxRecipientEnvelopeLimit
Get-TransportRule | where {($_.MessageSizeOver -ne $null) -or ($_.AttachmentSizeOver -ne $null)} | Format-Table Name,MessageSizeOver,AttachmentSizeOver

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Size limitDefault valueExchange Management Shell configurationEAC configuration

Maximum size of a message sent through the Receive connector

Transport service on Mailbox servers

Default <ServerName>   35  MB

Client Proxy <ServerName>   35  MB

Front End Transport service on Mailbox servers

Default Frontend <ServerName>   36  MB

Outbound Proxy Frontend <ServerName>   36  MB

Client Frontend <ServerName>   35  MB

Cmdlets: New-ReceiveConnector, Set-ReceiveConnector

Parameter: MaxMessageSize

Mail flow > Receive connectors > EditEdit icon > General > Maximum receive message size (MB)

Maximum size of all header fields in a message sent through the Receive connector

256 KB

Cmdlets: New-ReceiveConnector, Set-ReceiveConnector

Parameter: MaxHeaderSize

Not available

Maximum number of recipients in a message sent through the Receive connector

Transport service on Mailbox servers

Default <ServerName>   5000

Client Proxy <ServerName>   200

Front End Transport service on Mailbox servers

Default Frontend <ServerName>   200

Outbound Proxy Frontend <ServerName>  200

Client Frontend <ServerName>  200

noteNote:
If the number of recipients is exceeded in a message from an anonymous sender (for example, an Internet sender), the message is accepted for the first 200 recipients. Most SMTP messaging servers detect that a recipient limit is in effect. The SMTP messaging server continues to resend the message in groups of 200 recipients until the message is delivered to all recipients.

Cmdlets: New-ReceiveConnector, Set-ReceiveConnector

Parameter: MaxRecipientsPerMessage

Not available

Maximum size of a message sent through the Send connector

10 MB

Cmdlets: New-SendConnector, Set-SendConnector

Parameter: MaxMessageSize

Mail flow > Send connectors > EditEdit icon > General tab > Maximum send message size (MB)

Maximum size of a message sent through the Active Directory site link

Unlimited

Cmdlet: Set-AdSiteLink

Parameter: MaxMessageSize

Not available

Maximum size of a message sent through the Delivery Agent connector

Unlimited

Cmdlets: New-DeliveryAgentConnector, Set-DeliveryAgentConnector

Parameter: MaxMessageSize

Not available

Maximum size of a message sent through the Foreign connector

Unlimited

Cmdlet: Set-ForeignConnector

Parameter: MaxMessageSize

Not available

To see the values of these connector limits, run the following command in the Exchange Management Shell:

Get-ReceiveConnector | Format-Table Name,Max*Size,MaxRecipientsPerMessage; Get-SendConnector | Format-Table Name,MaxMessageSize; Get-AdSiteLink | Format-Table Name,MaxMessageSize; Get-DeliveryAgentConnector | Format-Table Name,MaxMessageSize; Get-ForeignConnector | Format-Table Name,MaxMessageSize

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Size limitDefault valueExchange Management Shell configurationEAC configuration

Maximum size for a message sent by Outlook on the web clients

35 MB

You configure this value in web.config XML application configuration files on the Mailbox server. For more information, see Configure client-specific message size limits.

Not available

Maximum size for a message sent by Exchange ActiveSync clients

10 MB

You configure this value in web.config XML application configuration files on the Mailbox server. For more information, see Configure client-specific message size limits.

Not available

Maximum size for a message sent by Exchange Web Services clients

64 MB

You configure this value in web.config XML application configuration files on the Mailbox server. For more information, see Configure client-specific message size limits.

Not available

noteNote:
The pickup directory that's available on Edge Transport servers and Mailbox servers also has messages size limits that you can configure. Typically, the pickup directory isn't used in everyday mail flow. It's is used by administrators for mail flow testing, or by applications that need to create and submit their own messages files. For more information, see Configure the Pickup directory and the Replay directory.
  • Maximum size of all header fields in a message file placed in the pickup directory: 64 KB.

  • Maximum number of recipients in a message file placed in the pickup directory: 100.

 

Size LimitDefault valueExchange Management Shell configurationEAC configuration

Maximum size of a message that can be sent to the specific recipient

Site mailbox provisioning policies: 36 MB

All other recipient types: unlimited

Cmdlets:

Set-DistributionGroup

Set-DynamicDistributionGroup

Set-Mailbox

Set-MailContact

Set-MailUser

Set-MailPublicFolder

New-SiteMailboxProvisioningPolicy

Set-SiteMailboxProvisioningPolicy

Parameter: MaxReceiveSize

For mailboxes:

Recipients > Mailboxes > EditEdit icon > Mailbox features > Mail flow section > Message size restrictions section > View details > Received messages section > Maximum message size (KB)

For mail users:

Recipients > Contacts > EditEdit icon > Mail flow settings > Message size restrictions > View details > Received messages section > Maximum message size (KB)

This setting available in the EAC for other types of recipients.

Maximum size of a message that can be sent by the specific sender

Unlimited

Cmdlets:

Set-DistributionGroup

Set-DynamicDistributionGroup

Set-Mailbox

Set-MailContact

Set-MailUser

Set-MailPublicFolder

Parameter: MaxSendSize

For mailboxes:

Recipients > Mailboxes > EditEdit icon > Mailbox features > Mail flow section > Message size restrictions section > View details > Sent messages section > Maximum message size (KB)

For mail users:

Recipients > Contacts > EditEdit icon > Mail flow settings > Message size restrictions section > View details > Sent messages section > Maximum message size (KB)

This setting available in the EAC for other types of senders.

Maximum number of recipients in a message that's sent by the specific sender

Unlimited

Cmdlets:

Set-Mailbox, Set-MailUser

Parameter: RecipientLimits

For mailboxes:

Recipients > Mailboxes > EditEdit icon > Mailbox features > Mail flow section > View details > Recipient limit section > Maximum recipients

This setting isn't available in the EAC for mail users.

To see the values of these limits, run the corresponding Get- cmdlet for the recipient type in the Exchange Management Shell.

For example, to see the limits that are configured on a specific mailbox, run the following command:

Get-Mailbox <MailboxIdentity> | Format-List MaxReceiveSize,MaxSendSize,RecipientLimits

To see the limits that are configured on all user mailboxes, run the following command:

$mb= Get-Mailbox -ResultSize unlimited; $mb | where {$_.RecipientTypeDetails -eq 'UserMailbox'} | Format-Table Name,MaxReceiveSize,MaxSendSize,RecipientLimits

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The order of precedence for message size limits is the most restrictive limit is enforced. The only question is where that limit is enforced. The goal is to reject messages that are too large as early in the transport pipeline as possible. For example, it's a waste of system resources for the Internet Receive connector to accept large messages that are eventually rejected because of a lower organizational limit. Make sure that your organization, server, and connector limits are configured in a way that minimizes any unnecessary processing of messages. You do this by keeping the limits the same in all locations, or by configuring more restrictive limits where messages enter your Exchange organization.

User limits between authenticated senders and recipients (typically, internal message senders and recipients) are exempt from the organizational message size restrictions. Therefore, you can configure specific senders and recipients to exceed the default message size limits for your organization. For example, you can allow specific mailboxes to send and receive larger messages than the rest of the organization by configuring custom send and receive limits for those mailboxes.

However, this exemption applies only to messages sent between authenticated senders and recipients (typically, internal senders and recipients). For messages sent between anonymous senders and recipients (typically, Internet senders or Internet recipients), the organizational limits apply. For example, suppose your organizational message size limit is 10 MB, but you configured the users in your marketing department to send and receive messages up to 50 MB. These users will be able to exchange large messages with each other, but not with Internet senders and recipients (unauthenticated senders and recipients).

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The following list shows the types of messages that are generated by Mailbox servers or Edge Transport servers that are exempted from all message size limits except the organizational limit for the maximum number of recipients that are allowed in a message:

  • System messages

  • Agent-generated message

  • Delivery status notification (DSN) messages (also known as non-delivery reports, NDRs, or bounce messages). However, you can use the ExternalDsnMaxMessageAttachSize and InternalDsnMaxMessageAttachSize parameters on the Set-TransportConfig cmdlet to limit the size of original messages that are included in DSN messages (hence, the effective size of the DSN message itself).

  • Journal report messages

  • Quarantined messages

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