Mail Delivery Failed: returning message to sender

[Andy.I]

Hello everybody, I'm new on here.

I suspect many people arrive on here looking for help and I am no exception! I have a particularly vexing problem which has me baffles and no amount of searching has yielded any answer.

I am running Thunderbird 91.3.2 (64-bit) on Windows 10 and have used it happily for a couple of years with hundreds of contacts. However, I now have recently experienced a problem when attempting to send email to two specific and unrelated email addresses. One is a new contact and the other is one with whom I have exchanged a few emails and this problem seems to have just started. Now when sending email to either contact I invariably receive the following bounce-back:

==
Mail Delivery Failed: returning message to sender

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

martin@xyzxyzxyz.co.uk
host sxb1nlsmtpcp-v01.prod.sxb1.secureserver.net [92.204.64.10]
SMTP error from remote mail server after end of data:
552 5.2.0 skAhmnpPHQKMm :: CPANEL :: Message rejected for spam or virus content ::
Please include this entire message when contacting support ::
v=2.4 cv=KM2fsHJo c=1 sm=1 tr=0 ts=61a8ac7f p=DtyTGJbPAAAA:8 a=I/10cnYL9IeEYSQyKmX4FA==:117 a=Tf8FmXrM+cnex4ie0iqLWg==:17 a=9+rZDBEiDlHhcck0kWbJtElFXBc=:19 a=IkcTkHD0fZMA:10 a=IOMw9HtfNCkA:10 a=yPW59B4Lm_zDAtYt2ooA:9 a=QEXdDO2ut3YA:10 a=__GtDX3R1_kA:10 a=ycG1OLHhaeIA:10 a=5U6lEk5n4T0A:10 a=B1KzvHMFUXzV8Rg9nqW4:22 a=w4s9L7PFYQJYVDaC5bu7:22 ::
100.00

Reporting-MTA: dns; n3plcpnl0008.prod.ams3.secureserver.net

Action: failed
Final-Recipient: rfc822;martin@xyzxyzxyz.co.uk
Status: 5.0.0
Remote-MTA: dns; sxb1nlsmtpcp-v01.prod.sxb1.secureserver.net
Diagnostic-Code: smtp; 552 5.2.0 skAhmnpPHQKMm :: CPANEL :: Message rejected for spam or virus content :: Please include this entire message when contacting support :: v=2.4 cv=KM2fsHJo c=1 sm=1 tr=0 ts=61a8ac7f p=DtyTGJbPAAAA:8 a=I/10cnYL9IeEYSQyKmX4FA==:117 a=Tf8FmXrM+cnex4ie0iqLWg==:17 a=9+rZDBEiDlHhcck0kWbJtElFXBc=:19 a=IkcTkHD0fZMA:10 a=IOMw9HtfNCkA:10 a=yPW59B4Lm_zDAtYt2ooA:9 a=QEXdDO2ut3YA:10 a=__GtDX3R1_kA:10 a=ycG1OLHhaeIA:10 a=5U6lEk5n4T0A:10 a=B1KzvHMFUXzV8Rg9nqW4:22 a=w4s9L7PFYQJYVDaC5bu7:22 :: 100.00
==

I can send both these contacts emails using Roundcube without a problem. From this I deduce the problem is within Thunderbird although I'm prepared to be corrected on this!

What puzzles me is that the error message indicates two recipients, namely my contact and 'rfc822'. It appears that rfc8222 is something to do with the message format. The other puzzling bit is that I read in the error message "Message rejected for spam or virus content" although naturally I have checked thoroughly for viruses and I am confident my PC is clean.

Can anybody shed any light on this or point me in the right direction, please?

Thanks in advance!
Andy.I

[DanRaisch]

Does the message include an attachment or a signature file? Is the message formatted in HTML? If so, does the message go through if you send it as plain text with no attachments?

[tanstaafl]

"SMTP error from remote mail server after end of data:
552 5.2.0 skAhmnpPHQKMm :: CPANEL :: Message rejected for spam or virus content ::"

This probably means that the SMTP server that you used with Thunderbird has a poor reputation (it is on a blacklist) . If you send the same message using webmail its not using that SMTP server even though it may be using the same host, so it can have a different reputation.

If you have another account with a different email provider try temporarily configuring the first account to use the second accounts SMTP server, and see if that works around the problem.

Try entering your SMTP servers name in https://mxtoolbox.com/blacklists.aspx and see if its on a black list. You might also try sending a plain text message with no signature and see if it has the same problem. If it gets rejected that means its even more likely its not the content of your message (unless it is truly infected by a virus) but the SMTP server you used.

Think about adding a Gmail IMAP account that you can use just for troubleshooting. Gmail has a excellent reputation for detecting spam. I suspect that if you send a message from that account it would arrive.

[Andy.I]

Thank you for your suggestions - I think with your help I have tracked down the problem even though I don't understand why it was happening.
I have a gmail account which works without issues or problems so I decided to look elsewhere and started with my email sig. Surprise surprise - no sig, no bounce!
It turns out that with my sig entered into the text box in Account Settings, emails bounce, but if it's defined in a linked text file, they don't bounce!
Originally my sig was set up as a text file but a while ago I noticed the link was periodically being lost so I changed to the text being entered as html in the box in Account Settings.
I have now recreated the text file for my sig and it appears to work - at least I don't get any bounce-backs!