I'm using getmail with ISPConfig 18.104.22.168 (should upgrade to 3.0.4...).
If you get mail from 'email@example.com' from another server somewhere else (I just tried the SimplePOP3Retriever) and want to store it on your ISPConfig server to user 'firstname.lastname@example.org', mail is bounced. And lost.
That's because ISPConfig use MDA_external as destination type while there seems to exist other destination type like Maildir (not tested). So the mail is bounced to the original external server and not to your ISPConfig server.
Many thanks for your work !