What is the antivirus, and what are its settings for scanning outgoing mail?

@ sfhowes

Thanks for your response. I'm using Windows Defender. I have tried turning off real time protection and then sending a message with a PDF attachment but it makes no difference.

I can't see that I can specifically stop scanning of outgoing mail?

It's not necessary, or advised, to disable real-time background scans, but as with all AV products, the TB profile folder should be added as an exclusion to WD settings:

https://winaero.com/exclusions-windows-defender-windows-10/

Help/Troubleshooting, about:profiles, to see the profile location.

Hard to say why the other TB setup doesn't see the same error when sending the same attachment. If the profile was corrupted, you could test by creating a new profile, adding an account, and send a test attachment. Create and manage profiles in about:profiles.

If the error is entirely due to scanning at the recipient's end, either by their mail provider or their mail and AV apps, then the issue must be corrected there.

I'm up for trying a new profile, but before I do that can I tidy up the existing profile(s)?

If I run "thunderbird -p" from the command line the Profile Manager dialogue displays but only has a default profile to chose from. However, when I look at the profile directories both roaming and local I get multiple profiles as shown on the attachment.

I have highlighted the default profile. Can I safely delete all the others?

Thanks,

If some profile folders don't appear in Profile Manager or about:profiles, they might be legitimate profiles, but since they aren't registered in profiles.ini, they can be deleted or removed from C:\Users\username\AppData\Roaming\Thunderbird\Profiles. To make one of these profiles appear in TB, create a new profile and Choose... a folder as the source of the new profile. If you don't choose an existing source, the new profile will be created with default, empty subfolders.

Hi,

Well I created a brand new profile, added an account and tried again. Same result.

Looking again at the changes, during the period since it last worked properly this machine has had a trial version of Acrobat Pro installed and removed, and I am starting to think this has left something behind which is somehow raising flags on all the PDF files which Thunderbird is unable to resolve. I cannot see any other differences between this machine and the other TB machine where it does work.

I will have to find a workaround for this problem. Much appreciate your assistance in this.

Thanks and regards,

If you've changed the app for pdf files, remove the Action for pdf in Options/General/Files & Attachments, restart TB, open a pdf attachment in a received message, and set the opening app in the resulting dialog with the current default pdf app. Then restart TB and see if that affects the sending of pdf attachments.

I'd already tried that, as years ago I encountered a problem with sending files with PDF attachments, which was solved by assigning "always ask" to files with .pdf extensions. When I first encountered this current problem I checked and found I had three different options for PDF files, and cleaned the others out. Sadly, it doesn't help.

I'm still suspicious about Acrobat, which in its default guise thinks that Outlook is the only mail client out there. Indeed, I can still only send a .pdf attachment via Outlook.

Back to the workaround!

Thanks and regards,

However, it has now got a lot worse! I now cannot reply to anything that has graphics of any kind in the original message! Same error message which seems to come from the recipient's mail server.

Is that another clue?

The error message in your original picture is coming from your mail server. so I would ask your mail provider. I don't know why they would block TB but not Outlook.

And I should have checked on my mail server!!!

Sure enough, there was a major change on April 21, and a search through the community blogs for BT / Yahoo users reveals a number of other users on the same platform (Windows 10, TB v78.10.1) have the problem.

I guess that is where I should continue this discussion!

Thanks for your help.