System:
Mail configuration was done with plesk.
It is not possible to send any mail to any reciepents on different domains.
Log contains:
qmail: 1553679224.737428 delivery 12: failure: Sorry,_I_couldn't_find_any_host_named_example.com._(#5.1.2)/
dig example.com or dig -t mx example.com resolves correct.
On Ubuntu 18.04 is systemd-resolved installed by default as local dns resolver.
Most of the linux tools can use this dns without problems.
qmail seems to query the dns in a different way like dig or other linux tools.
I found the problem by use dnstracer.
Doing
dnstracer -c -q mx example.com
returns nothing, because 127.0.0.53 is used.
Systemd-resolved is not a full dns server!
Doing
dnstracer -c -q mx -s 1.1.1.1 example.com
returns correct answer, because 1.1.1.1 is used.
So I linked the /etc/resolv.conf from
/var/run/systemd/resolve/stub-resolv.conf
to
/var/run/systemd/resolve/resolv.conf
and all works fine now.
You can also disable systemd-resolved completely and use resolv.conf in the old way.