Many thanks to Von-Austin See in SM support. In all the years I have never needed to open a ticket and hated doing so but he led me to an error message about "data transfer failed" in the SMTP log and upon googling I saw another topic which led me to my Watchguard firewall and the spamBlocker subscription service. I turned it off the problem went away, I then went back and reconfigured it as well as the routing config and all is fine now. Hallelujah, two days shot and four days of e-mails blocked but I'm happy to have this finally cleared up!
Thanks to all who helped.