IMAP migration (Internet Message Access Protocol migration) is a way to move email services from existing non-Exchange email servers to later versions of Microsoft Exchange Server, Exchange Online or Office 365.
Administrators begin IMAP migrations using the Exchange Administration Center or the Exchange Management Shell, which allow user mailbox content to migrate from an IMAP system to new Exchange mailboxes.
An IMAP migration process varies depending on which product is used -- since there is no one single IMAP server-- but the overall process for IMAP migrations to a cloud-based email service begins with the Exchange administrator obtaining the full name of the IMAP email server. Next, the admin uses a tool like Excel to create a CSV file containing details of each email account to be migrated, though the actual formatting will vary depending on the IMAP system. Then the admin establishes a connection between the source email server and the destination email system.
After the IMAP migration is complete, the administrator can redirect email messages directly to the new server, and then delete the migration batch to stop synchronization. At this point, the new email system is operational and the admin can stop the old email server.
An IMAP migration takes several hours to several days depending on the number of mailboxes, batch sizes and the amount of content in each mailbox.
Email messages larger than 35 megabytes cannot be migrated, and only up to 500,000 items per mailbox can be transferred.
To make an IMAP migration faster, the administrator can increase connection limits to the IMAP server and users can delete old email content prior to the migration. The administrator also can organize the IMAP migration with small, focused batches of mailboxes.
A staged (partial) migration is advisable when migration involves a larger number of mailboxes, or the process must be performed incrementally using smaller mailbox groups. Later versions of Exchange email servers must use cutover migration. This type of migration is limited to Exchange deployments of 1,000 mailboxes, or use hybrid deployments, which allow local and online deployments to coexist until administrators can organize migrations.