Comment 6 for bug 180551

So...

gethostname(2) is supposed to return an unqualified name (on Debian and Ubuntu at least).

The Postfix docs say that "The default is to use the fully-qualified domain name from gethostname()." but gethostname() on Debian and Ubuntu will never return a fully-qualified name.

hostname -f returns the result of getaddrinfo(2) on gethostname(2), as defined in hostname(1).

Postfix, when $mydomain is not set and when the result of gethostname(2) is unqualified, appends ".localdomain", when you instead expect it to use what "hostname -f" returns and thus to use the result of getaddrinfo(2) on gethostname(2).

Sounds like a question for upstream to resolve to me. Can someone check this issue is still current upstream and check for and/or file a bug upstream as appropriate, please?