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Plan to assess and reduce mailbox size for better Exchange Online migration performance


Topic Last Modified: 2014-03-10

Summary: Provides suggestions for reducing mailbox size during Exchange Online migrations and evaluating the impact of the migration on bandwidth.

The size of a mailbox, along with available bandwidth to the Internet, is a limiting factor in achieving a high migration velocity.

A common practice to reduce the size of the mailbox is to move email items out of the mailbox to an archive (for example, a .PST file) either manually or with auto-archive functionality. This practice is generally discouraged because when the user moves the mail items back to the new Exchange Online mailbox after migration, the Exchange Online mailbox will not allow the user to reply to these mail items. However, moving mail items into .PST files may be the only option to reduce the size of the mailbox in some instances.

When you attempt to reduce mailbox size, we recommend you do the following:

  • Delete or archive mail in the Sent Items folder.

  • Delete or archive all Calendar attachments.

  • Delete or archive Calendar items over 30 days old.

  • Delete or archive Inbox items over 90 days old.

  • Search for and delete attachments over 5 MB.

  • Disable the Journal.

  • Empty the Deleted Items folder.

  • Run the Mailbox Cleanup wizard from the Outlook Tools menu (Outlook only).

The number of days and file sizes described above are recommendations only. They may not be suitable for your organization.

Your organization will need to assess the number of mailboxes, mailbox size, and the rate of mailbox size growth in your existing environment. This information will help you evaluate the impact of migration traffic on your network, which must be considered when scheduling migrations.

If your organization enforces maximum mailbox size limits, this information is also important to consider when you define Exchange Online storage capacities. Your new Exchange Online environment should let all users store the same amount of data or more in their Exchange Online mailboxes. It might be necessary for users with extra-large mailboxes to move some of that content from their mailboxes to some form of offline storage, such as a Microsoft Office Outlook .PST file, to facilitate timely mailbox migrations.

In addition, you should also confirm if the capacities of your Exchange Online plan supports your existing mailbox inventory. Each Exchange Online plan supports different mailbox storage levels. For more information, see Exchange Online Limits.

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