Do you know if the same message or type of message worked in version 60?

Hi Wayne This problem only started after I upgraded to v68 earlier this week.

There's a preference change that works for gmail accounts, and it may fix your issue in the meantime: Edit/Preferences/Advanced/General/Config. editor and double-click mail.server.default.fetch_by_chunks to false, close Pref., restart TB. To force a re-download of a message, open the Message Source (Ctrl+U).

Thanks sfhowes but that didn't work for me. However I have experimented further. I use 1and1/ionos as my mail server and I also have a gmail account all accessed through Thunderbird. If I send an attachment to one of my 1and1 accounts the attachment cannot be opened but if I send it to my gmail account I can open it. This suggests that there is a problem with the way 1and1 interacts with Thunderbird.

I've just been reading a bug on issue that sounds like yours and person said the following was a workaround. https://bugzilla.mozilla.org/show_bug.cgi?id=1576584 initial comment mentions "When a message with attachment is opened some attachments are marked as "size unknown" and there's no way they can be opened or saved." read: comment 25 and 27

Please try it: Edit/Preferences/Advanced/General/Config. editor set 'mail.server.default.mime_parts_on_demand' to 'false'. double click online will toggle the value from True to false. close Preferences restart TB. To force a re-download of a message, open the Message Source (Ctrl+U).

Toad-Hall, that did it!! Setting 'mail.server.default.mime_parts_on_demand' to 'false' cured the problem. Well done. Thanks everybody for your help and support. Cheers

And should be fixed in 68.1.1 https://bugzilla.mozilla.org/show_bug.cgi?id=1579341