My sendmail is configured to deliver via LMTP (Unix socket listened to by Cyrus). So I don't think it uses Cyrus' deliver executable.
True, and you said it's only happening to some email, correct? I've had this problem once before, but it was ALL mail, and the problem was that the permissions on the socket were too restrictive.
I also wonder if it has anything to do with MailScanner. I run a two stage SMTP queue. SMTP mail is received and stored in /var/spool/mqueue.in and then MailScanner (www.mailscanner.info) scans it for viruses. If the mail is clean it moves the mail to /var/spool/mqueue which another sendmail process regularly flushes.
Possibly - queue re-injection issue? Check Mailscanner archives maybe?
How would I go about checking my 8 bit settings??
In the MAILER definition, check that sendmail is using 7 as a flag rather than 8. I'm away from my docs and my internet connection is painfully slow, or I'd be more detailed...anyway, good luck! -- Greig McGill