You need each of your gmail accounts to have its own smtp settings for sending.

If you use a gmail smtp server to send messages, google silently replace the From: address in your message to that associated with the smtp server set-up that you used to send it.

Both accounts use smtp.googlemail.com What should I be using? Thanks for your help

You need one smtp server for each incoming account.

Go to Tools|Account Settings and scroll down to the last item, "Outgoing Server (SMTP)".

Select it, review the smtp servers. Do you have two or more gmail settings there? If not you need to add a new one for the affected account.

If you already have two or more, select them one by one and review the settings. You need one for each gmail incoming account. Give each one a distinctive name under "Description" that tells you what account it relates to.

Finally, go to each accounts' incoming settings. Under "Account settings" you'll see "Outgoing server (SMTP)". Make sure each account refers to the relevant smtp server.

I think this situation arises because when you add an account, Thunderbird offers the smtp servers that are already set up, rather than encouraging you to set up a new one.

I don't see the same boxes as this is in linux. I have 2 options smtp.googlemail.com and smtp.gmail.com is this correct?

Which box(es) don't you see? The second of my images was taken on linux. The first is from an old windows installation. I'm using Linux right now and nothing shown there is very different here.

Did you follow my link at Tools|Account Settings?