Deleting an ACCOUNT from Mail should never delete the email itself from the server if you're using IMAP and all your email is actually on the server, in the INBOX and not in local folders. Even if your account is only accessed from one device, you are still not deleting the email itself from the IMAP account if you delete the account in Mail. The server only knows when there is a device/devices with the correct settings that connects and syncs email - it doesn't know (or care) whether there IS a device or not. You can have multiple IMAP email accounts all over the place and not connect to a single one - it doesn't matter. Whatever mail is sent to the email address is still there. That's how IMAP works. So something about this isn't right. I've deleted IMAP accounts dozens of times over the years - nothing ever happened to the email. However, that said, it's always a good idea to have a complete backup. Good how-to on using a backup here:
https://www.macworld.com/article/231354/how-to-recover-a-mail-folder-from-time-machine-in-macos.htmlDeleting the EMAIL from an account with IMAP will definitely remove it from the server - and from any connected local devices as well. Disconnected devices will not sync, of course - but as soon as they're connected again...poof. That's one of the reasons you can't just switch from POP3 to IMAP without taking some precautions first. Email in POP3 is typically downloaded off the server - maybe keeping 30 days or so of email on the server, but everything else is locally stored - in the Inbox. Switch to IMAP and the server looks at that Inbox and says "ok, there is 30 days of email here on the server...let's sync this puppy." And what's on the server initially, takes precedence. It happily deletes everything on the local device so that it matches what's on the server. Say goodbye to anything older than the 30 days you had from POP3. (of course, going forward, you're fine - stuff stays on the server unless you delete it locally, at which point it gets deleted (synced) with the server). The precaution to take in this case is to move all email out of the inbox (and sent folder) into local folders. Then after you've got yourself connected via IMAP, drag stuff back into the Inbox and Sent folders - but it's a good idea to sort it out, so you're not keeping things on the server that you really don't need or don't need access to from all devices. It just slows things down.
