Is it safe to ublock 587, SMTP submission port?
Yes, mail can be sent through port 587, so spam can be sent as well. That being said, most mail server using port 587 will also ask for authentication, so it should be much less of a problem. To be even safer, you could have a local mail forwarding server that require authenticaion and require all mail to go through this server.
Yes that port is safe as it requireds the user to authenticate before allowing the mail to be relayed.