Please explain what is happening when you try and send an email from this account. Error messages?

I get this message

I can send mails with the other addresses. Th one which doesn't work is one of my husband's. Of couse I have checked everything...

He receives mails though.

That's telling you that the authentication protocol must be TLS and it also advises you to check the email address. Hamburger Icon (top right)/Account Settings/Go to the relevant email account/Server Settings/Security Settings are on the right.

Thank you for your answers but I have checked all that...

Does the user name/email address in the 'From' section of your email (1) match the User name in your SMTP Settings (2) and the email address in your Server Settings (3)?

1 - Hamburger (top right)/Account Settings - click on the name of the account through which you are sending the email. On the right is the return email address called 'Email Address' unless you have put another email address in 'Reply-to Address'.

2 - Hamburger (top right)/Account Settings/Outgoing Server (SMTP). Highlight the server through which you are sending the email and the User Name is displayed at the bottom. It can be changed using the 'Edit' button.

3 - Hamburger (top right)/Account Settings - click on the 'Server Settings' under the account through which you are sending the email. The email address is in the 'User Name' field.

It's not unusual for an Email provider to change the servers - so doing that with the latest release of their details is worth doing.

I have checked, and the name for his SMTP account is mine, because I'm the one who has the account. If I put his name instead, I dont get my messages any more. What is strange is that he gets his mails. It's only sending the problem.

I have checked, and the name for his SMTP account is mine, because I'm the one who has the account. If I put his name instead, I dont get my messages any more. What is strange is that he gets his mails. It's only sending the problem.

How many SMTP entries do you have? Only one? You should have a distinctive SMTP Server for each account - the return email address must match the log in details or the servers can reject the traffic.

I have checked with thunderbird on my laptop and the servers are OK. I have added TLS and it works. I'm going to try to change the settings on the PC. Thank you