Thunderbird does not reject the password. It is gmail who are rejecting it.

If you have a pop gmail account or an imap account that does not use Authentication Method: OAuth2 then gmail will periodically deliberately block you. This forces you to logon to the gmail webmail account and reset the settings to allow 'less secure apps'. They state this very clearly here:

• https://support.google.com/accounts/answer/6010255?hl=en

This is because gmail wants you to either use their webmail account or to use imap and 'OAuth' as Authentication method.

However, if you change the incoming and outgoing smtp server settings for Authentication Method to 'Oauth' and restart Thunderbird, then you will be prompted to enter the password and then gmail will create an Oauth token stored in Thunderbird. From then onwards it will use that token.

Note: for OAuth to work you need cookies enabled.

• Select 'Menu app icon' > 'Options/Preferences'
• In 'Options/Preferences' left Pane select Privacy & Security
• Select checkbox 'Accept cookies from sites'

Thank you Toad Hall. The penny dropped after I posted my problem. If Thunderbird is unused for a while, Gmail switches the access to "insecure" apps to "off". I hadn't used Thunderbird for a month and remembered later that this had happened before. Once I switched my "access to insecure apps" to on, I could access my email. Thank you for getting back to me so quickly. Joan