recent as I understand it is a download "feature" . Why would you try and use it when sending mail out?

You're right, it is a download feature, but if you look at the attached screen captures you'll see that when sending an email TB automatically includes the word 'recent' plus quotation marks in the sender's address, and it seems that this is the basis of the problem (just since the recent update - it has worked flawlessly for years until now).

If 'recent' is removed from the email address in the Account settings there's no problem - except loss of the function provided by 'recent'

I think I might have solved it - by removing 'recent' from the Email Address in the 'Account Settings' window but retaining it in the 'User Name' in the 'Server Settings' window.

Time will tell whether the 'recent' function still works, but at least I can now send email using this address without evoking an error message

From the Gmail article:

"If you're using the same Gmail POP account with multiple email clients you need to enable recent mode in order to let each email client access all of the messages in that account. You can do that by replacing username@gmail.com with recent:username@gmail.com as the username in Tools -> Account Settings -> Server Settings."