re : Gmail address will work if I provide access for less secure apps (Not good)

If you have a POP or IMAP gmail account which uses 'Authentication Method' = 'Normal Password' then gmail considers that as 'less secure'. If usingAuthentication Method'= 'OAuth2' then gmail considers it more secure.

Please note the language terminolgy used by gmail does not actually mean using a good strong password is insecure. But using 'less secure apps' option does mean on occasion, gmail will deliberately block access until you logon to webmail and re select that 'less secure apps' option switched oof by gmail. Gmail claim they do it periodically for your safety, but it is more likely to get you fed up and believe the issue was

Basically, gmail would prefer you used their webmail access or access using OAuth2 authentication which is designed for easy access to various websites. It means gmail therefore glean more information about the types of website you frequent - data is worth money when it comes to marketing, so it not all only about security. The user gets an ease of use advantage - Instead of having various separate passwords to logon to various websites, you can just use the one gmail access. That is also the same one applied to your gmail acount in Thunderbird or any email client.

re: plusnet Outgoing Server (SMTP) settings

• Server name: relay.plus.net
• Port: 587
• Connection Security: STARTTLS
• Authenication Method: Normal Password
• User name: full email address

REstart Thunderbird to ensure all settings are saved.

Thanks for your explanation Toad-Hall, much appreciated. Unfortunately I have tried the settings suggested for the Plusnet account without any success - including a restart. It will be a shame if I have to change away from Thunderbird as I've been using it for ages and really like it. I've been resisting changing my email address as so many accounts are linked to it!

Problem solved - I tried the old trick of deleting and re-adding SMTP and by magic it works!!

I also have this problem with the outgoing emails with Plusnet. I tried the above suggestion on deleting and re-adding SMTP but still no luck, it worked on another laptop but now it doesn't. When it did I copied the settings but it did not work on the new laptop.

Sending of the message failed. The message could not be sent because connecting to Outgoing server (SMTP) relay.plus.com failed. The server may be unavailable or is refusing SMTP connections. Please verify that your Outgoing server (SMTP) settings are correct and try again.

Earlier I posted that deleting and re adding fixed this problem. Sadly it didn't. So I have given up and changed my email from a plusnet address as I was getting nowhere fast. Several hundred changes later......