I've submitted a support ticket but I'm just curious if anyone else has been experiencing their email bouncing back.

Today the problem seems to be with Yahoo Mail users. We only send a small number of business-related emails (e.g. order confirmation, customer service replies, etc) from Smarter Mail and no unsolicited emails. We have not received a single complaint but we are getting undeliverable email headers with the following:

Reason: Remote host said: 421 4.7.0 [TS01] Messages from xx.xx.xx.xx temporarily deferred due to user complaints - 4.16.55.1; see http://postmaster.yahoo.com/421-ts01.html

I'm assuming it's someone else on our mail server abusing email, sending spam, etc. that's causing this problem? It's really annoying because we've had to call customers to ask for alternate email addresses. Recently we also had problems with Smarter Mail, which Mark informed me was caused by an IP issue. I wonder if that is somehow related to this problem too...

I got a couple of these exact same errors myself on Friday of last week when sending to Yahoo users.  If you get any more info please post.

Mark says the issue should be resolved now. Apparently someone on the same mail server was sending out a lot of spam but their service has been terminated. My client has not sent any emails since this morning so we'll have to see what happens tomorrow.