Hello! I couldn't find any working solution for my problem online, but please don't yell at me if this question has been asked before. I'm using ISPConfig 3.1.1p1 on Debian 8 with Dovecot as IMAP server. I have configured a domain "xy . com" with a mailbox "[email protected] . com" and "[email protected] . com", the hostname of my server is "mail01 . foobar . com". Both mailboxes are only allowed to send emails, so "Enable receiving" is off for both of them, IMAP and POP3 are disabled. There is another (MS Exchange) mailserver configured for receiving emails, let's name it "mx01 . bigcompany . com". There is no MX record configured for mail01 . foobar . com. There is an MX record pointing to mx01 . bigcompany . com. When I send an email from "[email protected] . com" to "[email protected] . com" postfix replies with 550 5.1.1 <[email protected] . com>: Recipient address rejected: User unknown in virtual mailbox table; from=<[email protected] . com> to=<[email protected] . com> proto=ESMTP helo=<[...]> How can I tell postfix to disable local delivery for this domain? Sorry for the weird hostnames and email addresses, the forum thinks they are links and doesn't allow me to post links.