This is called SPOOFING and I don't give folk any direct answer to such things. Why? Because it can be used for evil bad things. So on your search try the words How to spoof my return email address?
Bob
When someone sends me an email to my email setup at my own domain name i.e. anythining@mydomain.com it is forwarded by the registrar to my gmail email, through email forwarding settings.
I then use windows mail to read my email, but if i reply to the sender, it is replied by my gmail email address, not my personalised domain name email.
How do i make it look like the reply comes from my personalised email and not my gmail account.
My registrar does not provide this service, it would have to be an external solution.

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