Postfix STARTTLS configuration

Hello, i've installed postfix on port 25 with free let's encrypt certificate, it works actually with sending messages through local services such as telnet, roundcube webgui, ect. But when i'm trying to add it inside my android smartphone, it says that this connection is not enough secured, please give me advice on how can i solve the problem of receiving push notifications on my smartphone from my own mail server, and send it back as well. Google passes the verification of IMAP connection but fails the SMTP part completely, i thing that i need to somehow implement STARTTLS on the side of postifx, but dntknowhow. Thanks with an advice.
Mail submission is typically done on port 587, not port 25. So make sure postfix has them both open.