-- My error says AUP#MXRT. AUP ==> [Shaw's] Appropriate U...

[mdk]

@Graham15 -- My error says AUP#MXRT.

AUP ==> [Shaw's] Appropriate Usage Policy

MX ==> mail-exchanger [server].  Is your employer's computer connected to a Shaw cable-modem, and are you specifying (in the mail configuration on your computer) server-names such as "mail.shaw.ca" or "smtp.shaw.ca" as the server to receive your message? Those Shaw mail-servers will try to connect to the "MX" server as defined by your colleague's domain, to deliver your message. Compare to you dropping a letter into a Canada Post mailbox. The letter is collected, and moved to the Canada Post warehouse in your city. If the destination address lists a different city, the mail is moved to the Canada Post warehouse in that city. Writing invalid city/province details on the envelope will cause your letter to be returned to you -- not moved to any different city.

RT ==> Hmm. Maybe a "routing" issue, such as no "MX" record for the mail-server defined for your colleague's domain, or an invalid "MX" record ?

What is the domain-name (the part after the "@") of your colleague's E-mail ID? With that information, the MX record(s) can be checked.

Finally, note that the "reputation" of each MX server is known.  Some mail-servers will reject any E-mail originating from a MX server that has a "bad" reputation, i.e., a compromised server that is being exploited by spammers.

[khensu]

This is a issue we have started encountering too. MXRT means they are rate limiting connections. I have also seen AUP#CNCT which is too many concurrent connections.  This is coming from the Cloudmark filtering service that Shaw uses. 

I work for a webhosting/email hosting company. A lot of clients send email to shaw addresses so this is a big concern. We have tried two current routes to send email to their MX, shw-central.mx.a.cloudfilter.net but so far both end up bouncing with the AUP#MXRT message.  We are now trying a third way. 

I am also a shaw customer. 

[mdk]

@khensu -- thanks for the good information. Your situation probably is being encountered by other large senders of E-mail, such as universities that have mailing-lists for each course, with many students not using their "@BigSchool.edu" E-mail IDs, preferring to use their "@shaw.ca" or "@gmail.com" IDs.

Can you contact Shaw, to increase the "daily-limit" on connections from your mail-servers to Shaw's mail-servers? Presumably, Shaw allows universities to have higher limits. Shaw does create "feedback" loops, sending messages back to the university's I.T. Department, when "too much" E-mail is being sent to Shaw IDs.