Is that after you click send? Are you running out of local disk space to store the attachments in the TMP folder?

There is no specific limit and any I am aware of. The only google responses I see are implemented in the SMTP server not the mail client.

Sorry I have not responded earlier. I was looking in the wrong place for a response to my question. Yes - it is after I click "Send". I have uploaded an image of the message I get. I have 100GB of free disk space on my C drive where my Thunderbird programme is and 531 GB free space on the drive where I keep my Thunderbird profile - with all the mailboxes and archives.