9front Mail(1) attachment with .docx

I had a receipt I needed to mail that was in .docx format. Docx is really a zip file, and when sending mail upas/vf flags it as suspicious. For some reason, the queue hung when sending, and I wasn't able to determine the cause: it would hang on connecting to the external smtp server. I was able to send the same attachment to a different e-mail address, so maybe it was a one-off, but I did get a bit better understanding of the mailing process on 9front. If the problem arises again, I'll know to check out /mail/queue/username/, and tweak /mail/lib/qmail, /mail/lib/remotemail, and /mail/lib/rewrite as needed to debug.