TOP shows nothing that would normally worry me.
Just stopping sendmail and restarting it a few minutes later did not help.
Yes, SpamAssassin, recently updated, is on board. Has been since the machine was set up 3-4 years ago and on the previous ones. No changes there either.
Yes, plain text mails with no attachments go through instantly, from same sender to same recipient.
When sendmail has that attachment hysteria, it defers all mails with attachments.
Routine fax-to-email mails, same as for the last dozen years, test mails with small attachments, basically anything, that has an attachment.
After stopping sendmail and apache, with a 5 minute pause before restarting them, sendmail works again the way it is supposed to, as if nothing had ever happened.
mailQueue shows some deferred mails, but nothing outrageous. When there is a net problem somewhere, then some of the daily mailing lists generate pretty scary mailqueues, but the server handles those without a problem.