Email notification config - Helo command rejected: need fully-qualified hostname

[TN22]

I'm working on setting up email notifications on my two TrueNAS Scale boxes (primary and backup.) Currently both are running Angelfish 22.02.3. One is working fine; test emails go off without a hitch. The other is giving the error -

Error: [EFAULT] Failed to send email: {'redacted@redacted.com': (504, b'5.5.2 < bkpnas >: Helo command rejected: need fully-qualified hostname')}

I've compared the config on both boxes several times and don't see a meaningful difference; is there a way to tell why the primary box is working but the secondary isn't providing its full FQDN?

((I checked /etc/mailname and both have the generic tnsbuilds01.tn.ixsystems.net value, so it's got to be pulling it from somewhere else.))