How to perform cross-platform migrations of the e-mails?
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.