Migrate your mailboxes
Copy messages over IMAP, prepare MX, SPF, DKIM and DMARC, and switch without losing a message.
The principle
Your messages are copied from one mailbox to the other over IMAP while the old service keeps receiving. You only switch the MX records once the new mailboxes are ready: no message is lost.
1. List your mailboxes
List the addresses, their aliases and forwards, and their size. Also spot what sends e-mail in the domain's name (website, application, newsletter tool): they must be allowed in the SPF record.
2. Create the mailboxes on the new service
Create each address with a temporary password, with its aliases and forwards. Don't change the DNS yet.
3. Copy the messages
With an IMAP synchronisation tool such as imapsync, copy each mailbox from the old server to the new one. It can be run again: only new messages are copied. Make a first pass several days before the switch for large mailboxes.
4. Prepare the DNS
Lower the MX records' TTL to 300 seconds a day or two before. Prepare the new values: the new service's MX, SPF, DKIM and DMARC.
5. Switch
Change the MX records, then publish SPF, DKIM and DMARC. Run the IMAP synchronisation one last time a few hours later to fetch the messages that reached the old server while the change spread.
6. Reconfigure computers and phones
Update the incoming and outgoing server in each mail program, or delete and recreate the account. Ask each user to pick a final password.
Check
Send a message from and to an outside address and check in its headers that SPF, DKIM and DMARC pass. In the client area, the domain health page flags a missing record.