What those messages are telling you is that Gmail sees your email address as insecure. The SPF and DKIM refer to settings that need to be in your server (not in Thunderbird). I suggest you share these messages with your email provider to get it fixed. Google (and many other email providers) have been getting tougher in past couple of years on security, and your issue has been posted here regularly as others have had the same problem. (Brief overview: Gmail sees your email address as johndoe@xxx.com, but also sees that the email account is hosted at zzz.com. The server at zzz.com has no indicators that your account is hosted there, so Gmail refuses the message.)

I was afraid that you would see it that way. The people who look after my domain have put an SPF in and say there is no more they can do, but i should talk to the people who provide my email The people who provide my email say everything is satisfactory. So in some ways a problem with Thunderbird was my penultimate hope!

I regret you are not getting the help you need. Here are two links to give a bit of a tutorial. You cannot do this yourself unless you have a self-hosted email account: https://www.cloudflare.com/learning/email-security/dmarc-dkim-spf/ https://www.howtogeek.com/devops/dkim-dmarc-and-spf-setting-up-email-security/