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.

That error is coming from your mail providers server. The only way to even begin to understand what limits that particular server operates under is to ask the relevant mail provider.