When does dovecot use sendmail_path?

I'm trying to plug up all the holes where local mail leaves our mail system without a DKIM signature. One avenue is bounce messages.

I wrote a small sendmail wrapper to add DKIM signatures, and defined it as the value for dovecot's sendmail_path. I presume Dovecot invokes this executable when it has to send a send message.

Under what circumstances does this happen? I tried to provoke a mailout by sending messages to recipients with full quotas and permission locked mailbox files, but could not generate any mail back.

Joseph Tam <jtam.home@gmail.com>