If someone has received an email that purports to come from you, but which you are sure you never sent, then it's likely that a third party has sent it apparently (but not really) from your account. There are techniques for doing this which I don't understand but it's fairly common: from time to time I receive emails like this, apparently from people I know, but which in fact they never sent. If, however, you have sent an email to someone and that person has received it, but it has apparently come from a different address to the one you thought you sent it from, then maybe there's a muddle with your identities in Thunderbird. Like most email clients Thunderbird can cope with several 'identities' or 'aliases' for each email account. There's more about that here: https://support.mozilla.org/en-US/kb/using-identities

"Using identities

When you create a new message, Thunderbird uses the default identity for the selected account as the message's From value. "

That didn't happen. The identity used was from a list of recipients to a previous message. It was both automatic and unintended. And, I might add, a little scary. If I can send a message from my account and claim it is from anyone who has ever sent me a message...

Thanks for your response, amanchesterman. There is only one identity associated with this account.

Maybe there's some ambiguity here, I should have made this clearer. When you send an email you send it from an address. This is something like 'john@email.com' -- the familiar format with an @ in the middle. The bit before the @ can be a name, or a name + symbols: 'john' or 'john97' etc.; but it can also be something meaningless like 'tuvxyz'. Associated with the address there is a display name. This is often a person's name -- 'John' for example. But it could equally well be a nickname ('Big Guy') or a title ('Personnel Director') -- it's up to you. When an email is sent, the sender is identified as displayname <address> -- i.e. the address (as described above) is included between < and > brackets. But in an email client like Thunderbird, the sender is shown simply as 'display name' -- i.e., it's 'John' or 'Big Guy' that appears in the 'From' column, not the actual sending address with the @ in the middle. Now when I read your question originally, I thought you meant that somehow an email had been sent from your Thunderbird with a different sending address -- i.e. a different email account with an @ in the middle. But reading your response now, I'm thinking that actually it went from your address (with an @ in the middle) but with a different display name than yours? Still scary, I agree, and worth investigating, but not as serious as someone hijacking your account. Go to your Account Settings. You should see a field that says 'Your name'. This is the display name: you can change this as much and as often as you like. Underneath it is a field that says 'Email address': this is the actual address (with the @ in the middle), the account from which the mail is sent. I hope this clarifies a bit, and may help you find out what went wrong.