Planning
The honest answer is rarely the popular one: the schedule belongs to the two mail servers, not to the tool. Every IMAP migration downloads a message from one side and uploads it to the other, and both sides count what you take.
The size in gigabytes matters less than the number of messages. A thousand one-megabyte messages move noticeably faster than ten thousand hundred-kilobyte ones: each message costs its own round trip, and that overhead barely depends on size.
The second factor is how many parallel connections the provider allows. Hence the rule: estimate from the slower of the two sides, because a fast connection of your own cannot help.
Published figures are rare, but Google has them, and they are stricter than people expect. Gmail caps IMAP sync with two daily thresholds per account: 2500 MB of download and only 500 MB of upload. Hit one and access is suspended — usually for an hour, at worst for a day.
That leads somewhere non-obvious: moving out of Gmail runs about five times faster than moving into it. A ten-gigabyte mailbox leaves Gmail in four days and arrives in Gmail in three weeks, and nothing can change that except planning for it.
Microsoft does not publish its Exchange Online thresholds, so quoting a number would be inventing one. Two migration limits are documented though: messages over 35 MB are not migrated, and neither are folders with a forward slash in the name. Apple publishes no IMAP session limits either; the documented figure is a 20 MB message size cap.
The practical takeaway: a five-gigabyte mailbox usually moves overnight, a fifty-gigabyte one takes a few days, and that is normal. The bad outcome is not slow — it is stopping halfway with no idea what made it across.
Five minutes of estimation before you start saves the classic disappointment of scheduling a migration for Friday evening and expecting an hour.
It helps to exclude folders you do not want on the new side: trash, spam and archive labels that duplicate the rest of the mail. On Gmail that is often half the volume.
It does not help to buy faster internet, pay for a higher plan, or run several copies at once. The last one usually backfires: the provider starts refusing connections, and the whole thing runs slower than a single stream would.
FAQ
Yes. Reading and sending do not interfere with copying, and anything that arrives after the start is caught up by a second pass.
Usually a daily provider quota, or a folder with a huge number of small messages. The first needs waiting, the second needs patience.
A local client keeps working as long as the client is running. An online migration runs on the server and does not depend on the tab.
No. A plan changes the allowed volume, not the speed: the mail servers set that.
A separate password for IMAP instead of your account one: who requires it, where each provider keeps it, and what to do when the section will not appear.
Labels versus folders and the All Mail folder: the number one cause of doubled volume and duplicates, and what to exclude before you start.
One rule: mail first, DNS second. The full sequence, including TTL, the catch-up pass and the window when messages land in both mailboxes.
Connect two servers, look at the size and decide. Or take the client and keep everything on your side.
Code on GitHub · built on imapsync · no sign-up