From my experience, when you send a message as if you are sending from another account, the from field is the account that you specified. There are some email clients that will say from “[email protected] via [email protected]” — most notably outlook, and yes this can hit spam filters — but, overall, it seems to work fine.

I don’t have a fix for the ones that don’t work, though, especially when using gmail accounts, as they are all on the gmail servers and you cannot specify the alternate account’s outgoing server.

Hope that helps!