Ah! Gmail?

With gmail, when you set up an account in Thunderbird, you create an incoming account using POP or IMAP that collects your incoming messges. You also need to to set up a corresponding outgoing SMTP account using the same login credentials - i.e. username (email address) and password, and make sure each incoming account is using the appropriate outgoing setting.

Even though a message may say it's from A when you're composing it, if it is using B's gmail smtp set-up, the message will arrive at its destination saying it came from B.

How many accounts do you have? How many of them are with gmail?

Go to Tools|Account Settings and go down to the last item in the list of accounts, "Outgoing Server (SMTP)". Select it and look at the list of outgoing servers, which will appear to the right.

How many gmail smtp set-ups do you have? Do they all have distinctive "descriptions" so you can tell which is which? I usually put in part of the incoming email address as part of the description. (The attached screenshot is quite old and shows what I had before I got smart and started using the incoming email address in the SMTP description. You'll see that the SMTP being used here tells me absolutely nothing about which account it "belongs to".)

Now go to each incoming account's settings and see which smtp server it is using. Are there any non-gmail accounts using gmail smtp? Are there any gmail accounts using another gmail account's smtp?

I have not yet followed your suggestions but I have 2 gmail accounts on TB . One is working perfect, the other gives the above problem. How is that possible ?

The symptoms suggest that both of your accounts are using the same SMTP server. Each account needs its own SMTP server.

Look at the "Outgoing Servers (SMTP)" section of your Account Settings. How many gmail set-ups appear there?