It sounds like your emails are being labelled as 'Spam' or you have remote content attached to an image or are getting an attachment from whatever is scanning your outgoing mail.

Servers scan incoming mail using their Anti-Spam filters. Some servers will not like the email and simply delete it, so it never gets to recipient Inbox. Some servers will label that email as 'Spam' and put in the recipient 'Spam' folder.

If your personal computer IP address or the outgoing server IP address has been blacklisted then a server may block/delete email. Spam labelling can occur for several reasons. Content looks like spam or spoof email. It might have too many links or remote content.

Some people have Anti-Virus products running that scan outgoing mail and they attach a file or additional content to the email.

Thunderbird is not the cause.

Let's see what happens if you send an email to me. I'll check content etc. Please choose an email that you know got that error message as shown in posted image and send same content to me in a new Write message. Email: mail at anje dot co dot uk I've written email address in that manner so that spider bots cannot read it from a public forum. But replace the 'at' with a '@' and the 'dot' with a full stop/period.

Just to let you know that I received all five emails. I individually scanned each email and each attachment and all scanned as good.

Are you sending emails for that particular email address via the correct smtp server?

I'll send email to you with more details as they could be sensitive.

It would seem you needed to check the outgoing server as the email address was being relayed. This may be the reason for some servers thinking the email was spoofed. Good to hear you are managing to send again.