mail not sent with attachments

mail not being sent with attachments; log with untranslatable error message generated: MailClient.Storage.Application.ItemOperationException: Cannot send message (Email body not accepted due to the following reason: "5.2.0 4aumgv25RWag84aumgKg2s internal error. AUP#O-1240         Any ideas on how to solve this?

As an addendum, this error  was propagated when intereacting with the server (actually the older version It seems to have begun after Bresnan began throwing a Certificate error. While their help line was very patient, they could not id the error. No SMPT port changes in em Client had any effect. The error only seems to occur when there is an attachment added to the email. Pure email seems to go through unaffected. All works fine when using the clunky Spectrum web mail - only when using a 3rd party client does the error appear.

Semi-solved. Seemed to be a certificate problem from the provider that caused the SMTP server to choke. I was able to get things working with both K9 Mail and Thunderbird. Unfortunately, no amount of tweaking allowed me to get em Client to communicate properly with the SMTP server. K9 clearly stated that it was certificate problem. em Client just provides the indecipherable error message. As always, I will have to go with what works and modify my recommendations to others.