Trevor,
The problem may be at your end. (Have you changed anything there recently?) My reply to your address bounced when sent from my sendmail server, which hasn't changed in a long time. Here's what I got in the bounce message...
----- The following addresses had permanent fatal errors ----- trevor@tecnopolis.ca (reason: 550 trevor@tecnopolis.ca... 451 4.7.1 Greylisting in action, please come back later)
----- Transcript of session follows ----- ... while talking to tecnopolis.ca.:
DATA
<<< 550 trevor@tecnopolis.ca... 451 4.7.1 Greylisting in action, please come back later 550 5.1.1 trevor@tecnopolis.ca... User unknown <<< 503 5.0.0 Need RCPT (recipient)
--v7HEwiKb114371.1502981924/copper.cs.umanitoba.ca Content-Type: message/delivery-status
Reporting-MTA: dns; copper.cs.umanitoba.ca Received-From-MTA: DNS; lockwood3.cs.umanitoba.ca Arrival-Date: Thu, 17 Aug 2017 09:58:37 -0500
Final-Recipient: RFC822; trevor@tecnopolis.ca Action: failed Status: 5.1.1 Remote-MTA: DNS; tecnopolis.ca Diagnostic-Code: SMTP; 550 trevor@tecnopolis.ca... 451 4.7.1 Greylisting in action, please come back later Last-Attempt-Date: Thu, 17 Aug 2017 09:58:44 -0500
On 17/08/2017 3:22 AM, Trevor Cordes wrote:
I just started getting some weirdness with some email sent to my mail servers. Greylisting "come back later" (451) messages are being interpreted (it appears) by remote MTAs as a 5xx level error, meaning they immediately abort send attempts and bounce the email! I have never seen this before.
I've seen it from two separate remote MTAs so far, but from reports I'm getting in, it's happening on more.
Here's what I get from umce3cip03.ad.umanitoba.ca:
Remote Server returned '554 5.0.0 <[50.71.247.87] #5.0.0 smtp; 5.1.0 - Unknown address error 550-'foobar@tecnopolis.ca... 451 4.7.1 Greylisting in action, please come back later' (delivery attempts: 0)>'
On my server sendmail log I see:
Aug 16 08:43:15 pog sendmail[14379]: v7GDhFav014379: Milter: to=foobar@tecnopolis.ca, reject=451 4.7.1 Greylisting in action, please come back later
So umce3cip03.ad.umanitoba.ca appears to be taking my 451 and turning it into a 500/554/510 permanent error.
I also see this from yahoomail, but that's a different situation because yahoo is always considered "broken mta" in milter-greylist and we must whitelist all of their servers... but, the usual/previous symptom was yahoo would keep retrying as normal, just with a different IP each time, thus never passing greylisting. And now yahoo is doing the same thing as umce3cip03.ad.umanitoba.ca above (when I haven't yet whitelisted the particular IP that day, grrr):
Sorry, we were unable to deliver your message to the following address. foo@tecnopolis.ca: 550: foo@tecnopolis.ca... 451 4.7.1 Greylisting in action, please come back later
...