UPDATE: The issue is now resolved. When the email reputation went bad the mail server was moved to a different IP address that had a good reputation. The spam hadn’t been stopped so after a day or so that also had it’s reputation battered. BUT…in the meantime the original IP address had time to repent as it wasn’t sending any email at all, much less spam. Suspecting this would happen the email server was put back to the original IP address and it did come good. Email is back to normal. Apologies for any hassles.
There is a system of rating email server reputation based on the quality of the mail they send. Too much spam and you get a bad reputation. This is all evaluated automatically by some servers somewhere…there are lots of them that do it.
What is happening is that due to a WordPress site infected with spam sending script, servers have decided that the 123host mail server deserves to sit in the naughty corner.
Some ISPs use the naughty list to say “we won’t accept mail from you until your behaviour improves”. That’s where we are at right now. Note that it is only some ISPs. If you are getting bounced emails, maybe the people you are sending to have their email hosted there.
I am pretty confident the spam source has been stopped and things will return to normal soon. I am working on this with the data centre right now.
I will update this when I know more.