TT991 Error

[Ulminbore]

I know there are already several discussions of this topic, but I've seen none with a resolution that I understood.  Can someone help? A long-term e-correspondent of mine has suddenly started having his emails to me bounced with the following message...

Except that this message-system would not accept the cut-and-pasted copy of his bounce-notification "due to an Invalid HTML"...  So I've attached it as a pdf

Ulminbore

[Gondola]

I've removed your attachment as it was revealing personal data.

The error message is slightly misleading as the sender is being told that the delivery error is permanent. Not quite true.

The error code TT991 is a temporary error and what the message is indicating is that delivery is being delayed because there's something suspicious about the sender or the mail message. TalkTalk's CloudMark spam protection measures require the mail to be deferred for scanning. Deferral means delayed delivery. So you may get the mail message if it passes scanning or the mail message may not pass and could therefore be dropped.

A TT992 error is a fatal error and if that message is received by the sender then that is a permanent rejection of the incoming mail.

TalkTalk uses industry standard SPF authentication, DKIM signing and the DMARC policy of the originator to help determine if the sent mail is fully authenticated.