This question already has an answer here:
It's all in the title, but I'll spell it out:
Let's say my primary Gmail address is
firstname.lastname@example.org, and my alias is
I have my personal mail server forwarding mail addressed to
email@example.com and I'm using my SMTP server to send mail via
firstname.lastname@example.org using Gmail.
Someone sends me an email: Crab party inquiry to
email@example.com and I check it in Gmail. I hit reply. I want the "From:" to be automatically set to
firstname.lastname@example.org but by default, it is set to
email@example.com. I normally want to send mail and reply as
firstname.lastname@example.org except when the mail is addressed to
email@example.com. Is there a way to do this?