Unusual SPAM/discarded message on outbound mails
On a 6.0.9 install that's been running fine for some years now (clearly :-), I'm suddenly having a problem with some outbound email originated from the web UI.
A message to one recipient with CC to three others, on 3 different domains (yahoo, 2 gmails, and a corporate email service) is generating 4 individual bounce messages in the servers maillog, which look like this:
Jun 20 10:43:45 benjamin postfix/smtp: 88B611F00302: to=<firstname.lastname@example.org>, relay=127.0.0.1[127.0.0.1]:10024, delay=4.3, delays=0.01/0.01/0/4.3, dsn=2.7.0, status=sent (250 2.7.0 Ok, discarded, id=05742-15 - SPAM)
Each differs primarily in the to= address; the rest of the message is mostly identical, except for the ID. Notwithstanding "sent", the recipients don't seem to be getting the messages. Notably, even 250 2.7.0 is identical, which I would not necessarily expect if these were remote messages being perpetuated in my local log.
I've checked the server IP against a couple RBL lookups, and MXtoolbox's mail server checker; it's not an open relay, and no one has it blocked.
I can originate mail, apparently, to these recipients individually without getting the discarded message. The email in question has no attachments, and only a 2-level email forward cascade in it's body.
I'm a tad confused here; why would my own mailserver be marking *originating* mail (not even coming in via SMTPAUTH, say, from my cellphone's k-9) as spam, and dropping it? Any ideas?
Update: did check zmcontrol status, everything's marked as running.
Update: Happening on inbounds as well
A message from one of the people to whom the outbound was sent; hard drop, didn't even make it into my junk folder:
Jun 21 10:56:48 benjamin postfix/qmgr: C300F1F00156: from=<email@example.com >, size=133361, nrcpt=1 (queue active)
Jun 21 10:56:49 benjamin postfix/smtpd: disconnect from X3PPIR013.firstdata.com[220.127.116.11]
Jun 21 10:56:52 benjamin postfix/smtp: C300F1F00156: to=<firstname.lastname@example.org>, relay=127.0.0.1[127.0.0.1]:10024, delay=5, delays=0.93/0/0/4.1, dsn=2.7.0, status=sent (250 2.7.0 Ok, discarded, id=11339-08 - SPAM)
Jun 21 10:56:52 benjamin postfix/qmgr: C300F1F00156: removed