viktor,thank you very much for your input. it helped me to understand the issue and then solve it.
since this 'special' host is the local DNS server (dnsmasq), a reverse lookup returned this weird hostname.
so i had to add the DNS server to the mailhub's /etc/hosts, and the issue is solved.[1]
greetings...[1]: i don't like this solution. instead i'd like to configure dnsmasq to return the fully qualified hostname of the DNS server host, but that's OT here...
