I have no idea what is causing the problem, but I'll ask a few questions anyway: - Can you identify any changes that immediately preceded this activity in TB? - Does this happen in troubleshooting mode (no addons or other settings)? - Do you see an error message? If not, how do you detect the problem? thanks.

Thanks for your response David. Here is what I found after sending the query this morning; I removed gmail account, then added it again but picked POP3 protocol (if that's the correct term?) instead of imap.gmail.com (I did't think to try that before). Carried out 4 simple tests in sending empty mail then some text, then over 946 characters and finally picked an 11.7Mb pdf file, all transmitted with little effort. Seems that POP3 will allow sending attachments but IMAP did not. That took only a few minutes to carry out and I changed nothing else, simply chose to use POP3 instead of IMAP. I have left it alone since then but will send more files to test further. In answer to your questions: - changes? I have always accepted Ubuntu updates as they are issued, no matter what version I use,

 (the problem was apparent late last year, cannot recall attempting to send attachments after the beginning
  of the year)

- I haven't used troubleshooting mode and I'm unsure what addons are running or installed, - error message is: dialgue window showing smtp server timeout, this is after a few short minutes.

  (the 'sending email' window sits mid screen showing full progress bar but does not complete)

I was using Ubuntu 16.04.3 LTS since it became available and have always used it because I like the way it behaves. I chose to re-install the same version when the email problem became apparent but that made no difference. So I tried later versions of Ubuntu but still no difference. After your response I thought about the past versions of Thunderbird, I'm sure I recall that Thunderbird automatically set POP (or POP3 ?) protocol in the past, It seems that things changed at some point. It may even be months after any affecting update changes, that I realised the problem because the last email I sent with attachments may be at the beginning of 2021. For now, it seems that POP3 will allow send attachments, but why won't a supposed new protocol allow it?, at least on my system? Again, Thanks for response David. I hope this input will help anyone with any possible troubleshooting path.

IMAP and POP are both incoming protocols. ALL outgoing mail is sent using the SMTP protocol.

This may not help, but you can adjust the smtp timeout in config. editor by modifying the value for

 mailnews.tcptimeout   the default is 100   

Config editor is at Preferences>General . I do know gmail is picky on what attachments it handles.

I changed the 'Server' protocol back to imap, on the same day that I tested sending attachments with the Server setting as POP3 and can still send emails with attachments. Seems like odd behaviour of the, some or all, software involved. I have not altered the outgoing smtp server. So, as it was: - I could not send attachments with server set to imap, - changed from imap to POP3, - could then send any size file attachments, - changed server back to imap, - and I can still send any size file attachments. I did change the timeout setting to 500 before changing the server protocol, but it made no difference, I set it back to 100 after a few test send attempts. I'm not sure what made the process work properly, all settings are back to what they were when I could not send attachments. So for now I'll call it RESOLVED.

Hallelujah  :)