The mb2freedb gateway is going to be hosted at 188.8.131.52 (musicbrainzvm2.osuosl.org). Please allow this machine to send mail via the MB mail servers.
Logically this should use the 'smarthost' personality of the mail server, but that's only available on a private IP.
The right way to fix this would be:
Can also be used by machines with dynamic IPs of course.