I have several @mac, @me, etc. type email addresses I use in Mail. I also have one @gmail address that i rarely use but it is active.
I have my Mail Preferences so I create messages in only an @mac address
This weekend I received an email that was addressed to my @mac.com address. It was sent by an @gmail.com person
When I Replied to the person I discovered that the Reply was being sent From my @gmail.com address. I truly expected it to be from my primary @mac address as that was the address of the incoming email.
This is not the first time that messages had gone out with the @gmail address when I had no plans to do it
We suspect (know?) that Google doesn’t always play nice with Apple Mail. I think this is another example
My “theory” is that If a reply is being sent to an @gmail address Google manages to find an @gmail account to send it from
So, my alert is to look at your FROM address when replying. It may not be what you thought it would be