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Maximum Number of Connections to Your Mail Server


Applies to: Office 365 for professionals and small businesses, Office 365 for enterprises, Live@edu

Topic Last Modified: 2011-12-02

The Number of mailboxes to migrate simultaneously field specifies the number of connections to your on-premises mail server available to migrate e-mail to cloud-based mailboxes. The number of actual connections will never exceed the number you specify in this field.

For example, if the value is set to 3, the default value, you can migrate up to three mailboxes at the same time until all the mailboxes in the migration batch have been migrated. The maximum number of mailboxes that you can migrate simultaneously is 50 for all migration types. However, you have to use Windows PowerShell to specify a value larger than 10. For instructions on how to do this, see Increase the Number of Mailboxes to Migrate Simultaneously.

This setting is enforced only during the initial synchronization when you migrate a new batch of mailboxes. It doesn't apply when Exchange Online retrieves new messages sent to users' mailboxes in the on-premises messaging system or when you delete a migration batch.

The more connections you have to your mail server, the more mailboxes you can migrate at one time. This reduces the amount of time it takes to migrate a batch of mailboxes. However, these additional connections can consume your Internet bandwidth, negatively affecting network performance and users' ability to access Internet resources. As a general rule, use as many connections as you can to speed up the migration process.

How do you balance overall migration time and Internet bandwidth? It's a good idea to run several test migration batches, using different settings in the Maximum number of simultaneous connections field. This can help you find the best setting for you.

Try running test staged Exchange or IMAP migration batches of the same size, using different settings in the Number of mailboxes to migrate simultaneously field. Run the test batches at a similar time of day, and then compare the total duration for each test batch and any perceived impact on network performance and Internet connectivity. You may need to run several test batches to determine the optimal number of simultaneous connections.

If a particular test batch severely affects your network performance, just stop the migration, adjust the connection settings, edit the CSV file to remove mailboxes that have already been migrated, and then start a new migration batch.

Running test migration batches can also help you calculate how long it will take to migrate all your users' mailboxes.

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