Question
How to perform cross-platform migrations of the e-mails?
Answer
Full-featured cross-platform e-mail migration on a server level is not yet supported.
You may vote for such a possible feature on the UserVoice portal. The top-ranked suggestions are likely to be included in the next versions of Plesk.
As a workaround, it is possible to use the Site Import extension for each domain and mailbox. Currently, the Plesk Site Import extension allows transferring domain's databases, files, applications, and email on a per-mailbox basis.
Alternatively, use 3rd party IMAP migration tools to copy email content, or copy it manually.
Warning: Such functionality was not tested in Plesk and would be performed on your side and at your own risk.
Additional information
Site Import
How to use Mail Importing feature in Plesk?