I am not using the gmail smtp.
Then you may not understand the problem. We all know how to "relay" under "normal" circumstances.
Yet, it appears that you have to create an app password and use that as the password for your smtp auth. You certainly wouldn't want to use your regular password that you log into your gmail account with.
Sure. So what is the "app" that I claim needs the password? I don't think this is an "AppPassword" problem, but I would love to be proven wrong.
( ... several minutes later ... )
Dude, you might be right. It might be an "AppPassword" problem. Apparently "sendmail" and "postfix" qualify as applications that GMail can identify. When I asked "gmail apppassword for postfix" I found: https://www.systoolsgroup.com/add/postfix-to-gmail-account/
In deference to my ignorance, I will suspend further invective until I have tried what the above instructions tell me to try.