pop3 hosting transfer/multiple branches

brainstorming this idea...
We have a client that uses pop3 or imap with a hosting company and they want to change email providers. The client has about 20 branch offices nationwide, is a 24/7 operation with a mix of Outlook 2010-2016, none of the users are IT literate,
what is the best way to do this transition, with the least amount of downtime?
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David FavorLinux/LXD/WordPress/Hosting SavantCommented:
site:github.com imapsync provides many versions of imapsync, which is the goto tool for doing this.

You just aim it at a source imap server + a target imap server + clone the source server to the target server.

And... if you email stores are large, this may take a long while to complete.

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JohnBusiness Consultant (Owner)Commented:
You need to provide bullet proof documentation on what the parameters are for the new mail system. Use Screen shots that you annotate to make things easier.

Test the documentation on a willing but not too-technical user and see if they can change the account.

If you screen shot each, every and all steps numbered in sequence, they will be able to do it.
First setup the same accounts on the new host site. Add MX records in your domain host with lo priority pointed to the new e-mail host servers. Setup additional mailbox in Outlook profile (for the new host ) for each client. 2-3 days later change the MX records priority, new host with higher and old with lower. Once e-mails will stop going to the old host than slowly you can start the e-mail transfer and closing accounts with old host, if your clients prefer old e-mails available in the new mailbox.
David FavorLinux/LXD/WordPress/Hosting SavantCommented:
Accepted two answers which seemed best.

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