Hi Curlylewis,
Which type of email account are you using? If you are using a POP3 or a IMAP account, please follow the steps below to check if you have set the "Reply E-mail:" option to another email address:
1. Click File tab > Account Settings > Account Settings.
2. In the Account Settings dialog, highlight the email account which has the issue, and then click
Change button.
3. Click More settings button in the Change Account wizard.
4. Under General tab, check if you have set the "Reply E-mail:" option to another email address.

If this is the case, clear the email address in the Reply E-mail box and then test the issue again.
Regards,
Steve Fan
TechNet Community Support