SOLVED RE: IPv6 enabled sendmail does not send to domains with IPv4 only MX which results in “config error: mail loops back to me”

[Thorsten]

It wasn't a sendmail problem:

My hostname was with with domain "hostname.my.domain" not just "hostname" so the resolver in search for the non-existent AAAA of mx00.emig.gmx.net returned mx00.emig.gmx.net.my.domain which resolved to my local IPv6 address and sendmail tried to send the mail to itself.