So I've recently help migrate two different companies from older IMAP/POP3, and Exchange 2010 email systems.  There are some significant differences and things you can run into when migrated from these two very different email systems.

While Email is Email, IMAP/POP3 is very old technology and requires more work to be done preferably using the end users workstation.  With Exchange email systems a lot can just be done being logged into the server.

With IMAP/POP3 the pst files are located on the end user's workstation, you can collect them or just user the user's pc to upload them to Office 365, or from a central location- copy them up to Microsoft Azure and import them.

With Exchange Servers you can export all mailboxes to pst's with a command.  Then upload them to Microsoft Azure and import them, or use the end user's outlook.  Depending on the amount of email different approaches make the best since, or a hybrid of the two.

Note that when using Microsoft Azure the mailbox will be set with retention hold and email won't be deleted or archived if a retention policy is in place.