Dear all,
I know, that the topic is old. But I’ve had a simular problem and the info from @robgnu with the /etc/postfix/transport.mysql solved my problem.
And if someone else has a simular problem as well, maybe this info can help.
First of all my system: I installed kopano on a debian-10. I also installed fetchmail. I created user with mailaddress from my internal domain name eg: user1@mydomain.local and user2@mydomain.local
and some user with external mailaddress eg: user3@mydomain.de and also user4@gmx.de
For the internal user it works but for the user with external mailaddress fetchmail couldn’t deliver mails.
I tried fetchmail with kopano-dagent and without but no success.
Then I tried the mentioned tipp with transport.mysql - and it works !
I have to enter the external mailaddress in my /etc/postfix/virtual - but then it works.
And for me, it works better than expected, then: if I send a mail from user1@mydomain.local to user4@gmx.de the mail stays intern.
That it what I wanted !
I just want to describe it, if it someone else can help.
Best regards