Where to set hostname for SMTP HELO

I’m trying to configure mailing via an external SMTP server.

The SMTP host is using Postfix and rejecting email because the HELO does not have a fully qualified hostname - it’s just showing up as “akkoma”.

reject: RCPT from unknown[redacted]: 504 5.5.2 <a
kkoma>: Helo command rejected: need fully-qualified hostname; from=<redacted@redacted> to=<redacted@redacted> proto=ESMTP helo=<akkoma>

Where is this set?

(Update: I can see that in Swoosh, one can set a “hostname” in config: Overview — gen_smtp v1.2.0 . But this isn’t exposed in the admin frontend? Have been using database config and would prefer to be consistent if possible.)