Gmail thinks itself is a less secure application
Gmail does not allow itself to send via another Gmail acocount without allowing less secure apps to access the account
It was about 6 moths ago when I no longer could reply to email forwarded to me from another Gmail account. I had set it up to send via external email server and the "external" email server was smtp.gmail.com. I searched for a solution not wanting to lower the security on my second account. I have not yet found a solution other than allowing less secure apps.
/The Tenka Team