Are either of your accounts with Gmail?

satcal said

It recently started happening to me. i.e. I get an email addressed to my business email and click reply, send it and then I get THEIR next reply addressed to my personal email! When I go back to check, I see it was sent from my personal email!

Yes, all gmail. I have about 4 accounts

Gmail has an interesting quirk. It may also apply to other email providers, but I have never tried seeing up multiple accounts on other services.

If you have 4 Gmail accounts, say, A, B, C and D, then each needs to have its own specific SMTP server settings. If you have set account A up then added B, it's quite likely that Thunderbird will have offered you the SMTP you set up when creating account A. And while it doesn't show when composing a message, if account B is using account A's SMTP, the message will appear to have been sent "from" account A. And a reply to this message would be addressed to your account A.

The solution is to ensure that you have 4 SMTP server settings, for accounts A, B, C and D, and each account is set to use its appropriate SMTP server. It's important when creating the various SMTP server settings to name them so you tell them apart.

Thanks a lot! I wasn't sure how to do what you said about setting up 4 different ones but when I went into the SMTP settings I saw the username was not set to the default account! I changed it to the correct one and it seems to have worked. Again, thanks for taking the time to help.