IMAP migration (Internet message access protocol Migration) is a method of transferring email services from Exchange email servers to a newer version of Microsoft Exchange Server, Exchange Online or Office 365.
Administrators start IMAP transfer using Exchange Administration Center or the Exchange Administration Center and Exchange Management Shell. Exchange Management Shell allows user mailbox content to be transferred out of the IMAP system to the new Exchange mailboxes.
An IMAP migration process differs depending on the product utilized -- because there isn't a single IMAP server. However, the general process is the same. IMAP migrations to cloud-based email services start with the Exchange administrator getting the complete names of each IMAP mail server. The administrator then uses an application such as Excel to create a CSV file with the information of every email account to be transferred, although the formatting of the file can differ according to your IMAP system. After that, the administrator connects the email server that originated and the email server of the destination.
When the IMAP migration is completed, admins can direct emails straight to the server and remove the batch of migration to end the synchronization. The new email system is fully operational, and the administrator can shut down the old server.
An IMAP migration can take anywhere from a few hours to a few days, depending on the number of mailboxes, batch size, and amount of content contained in each mailbox.
The size of email messages over 35 megabytes can't be transferred, and only 500,000 mailbox items can be moved.
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