There is a lot of that going around. It seems to be a queue related problem. For some mysterious reason SENDMAIL puts stuff with attachments into deferred status into the queue. Even a mug-shot in the signature block can cause that. Plain text test mails get through OK.
Total mystery. Bad Sendmail update?
So far, manually weeding out the queue and rebooting Apache and Sendmail a few times gets the mail moving again, for a week.
I asked about weeding out the queue in a different post, but it is still waiting for the Honorable Postmaster to post it.