We had a mailing go out to 231 recipients, and it completed within the same minute it started. Looking at the report today, it shows that only 131 were successfully sent, the others all show as "unknown." I looked into /var/log/mail.log and can confirm that I don't see any lines of trying to send mail for those contacts that show as unknown. I also see records in the civicrm_mailing_event_queue table for what I believe are the same contact_ids that are unknown. Not sure that is related.
CiviCRM is set up to use the standard php mail() function.
I'm at a loss of even where to look for the issue. It just seems like CiviMail stop trying to send or something. Is there somewhere I can look to find some error? Some kind of limit we reached?