Thunderbird is only the messenger here.The email was sent to your email server, your email host having decided that the email id without the dot belonged to you. All Thunderbird did was move it from your server to your Thunderbird email account.

Thanks for your advice. But how do I stop it? Is it malign? Are my emails being diverted?

It may or may not be a 'feature', depending on your perception and your email provider. For example, it may be that your email provider allows flexibility in your email id, allowing for aliases. For example, you may want to pretend to have a different email id when subscribing to a newsletter and having the ability to eliminate the dot allows that and you could create a filter to manage it. Another example: gmail allows users to add -1, -2, etc. to email ids, yet they all still come back to the main one. So, the flexibility can be a plus, allowing for use of an alias. I suggest you check with your email host to see what other options exist. For example, if your id is john.doe, we know it can be johndoe, but could it also be j.ohndoe, johnd.oe, john.doe-1, etc. To me, this is a feature that you may find useful. It also has possibility of spammers, but I see that as minor, because spammers already know of your real id for them to attempt to modify it. There, too many words. I think your email is OK.

Thank you again David. My problem is that I know that there is someone else in my city who has the same forename and surname as as I do. I suspect that forename/surname, without the dot, is his email address. And I fear that he may be getting my emails in the way that i am getting his. Do you think this is not worth bothering about? Thanks again. DS

I think you're safe. He would be using a different email provider, so the letters to right of @ would be different. He isn't using your email provider, or the email host would have detected it.

Many thanks David. Quite reassuring!