Maybe if you post your SMTP settings, some suggestions will come forth. Thank you.

SMTP SERVER SETTINGS Description: BT Mail Server name: mail.btinternet.com Port: 465 (Default 465)

SECURITY AND AUTHENTICATION Connection security: SSL/TLS Authentication method: Normal password User name: anniep4@btinternet.com

I noticed that my husband's Thunderbird settings were identical except for the Description - his said BT Yahoo so I tried changing mine but it made no difference.

It may say BT Yahoo, but that is a legacy from years ago when BT messed around with outsourcing their email. I gather it was something of a disaster and they went in another direction fairly shortly after that, as far as I am aware anyway. But I also understand there are still the odd BT and Virgin subscriber that still have yahoo mail accounts, apparently they were allowed to elect to stay with yahoo. Your husband is not likely to be one of them if you share server settings beyond the name.

BT publish the following settings which appear to match up to your settings you posted.

SMTP settings
Outgoing Mail Server: mail.btinternet.com
Port: 465 (this may not be automatically populated on selecting SSL, so you'll need to check)
SSL Encryption: Enabled (but not STARTTLS)
Authentication: PLAIN
Username: your email address including the @btinternet or @btopenworld.com part
Password: your btinternet or btopenworld password

We are probably looking at another issue than the actual settings based on what is known as of now. My personal goto when email connections go sideways is the antivirus program. These ubiquitous products have a habit of short cutting and essentially hacking to do their job with frequent poor outcomes for things like mail that are no longer "mainstream"

Do you and your husband share a computer? What about a user account on that computer? What antivirus do you use?

My guess is you are not sharing with your husband and that your antivirus is scanning outgoing mails and issuing self signed encryption certificates which Thunderbird does not natively support. This view is tempered however in that you apearv to be getting mail and antivirus issues usually relate to all email not just one way.