Troubleshoot Mail Server Using DigitalOcean Tutorial?

I followed the tutorial https://www.digitalocean.com/community/tutorials/how-to-configure-a-mail-server-using-postfix-dovecot-mysql-and-spamassassin to try and get a mail server running at home. I’m on Comcast, however, who blocks port 25, so I added this into the config in order to use the Comcast relay. This is the only thing I did differently.

relayhost = [smtp.comcast.net]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options =

I then made a file /etc/postfix/sasl_passwd and put:

smtp.comcast.net        COMCASTUSERNAME:COMCASTPASSWORD

As far as I’m aware, this should work, but when I put in the details in Gmail it says it can’t reach the server. As for Gmail, I have:

SMTP Server: mydomain.net
Port: 25 [Since I'm using the relay, I'm assuming my end should be configured to 587 and outside should be pointed to 25, although I did try 587 on Gmail, too, to no avail.]
Username: email3@mydomain.net [I have 3 test accounts]
Password: [That account's password]
And I have "Secured connection using TLS" bubbled.

How should I go about troubleshooting this? Thanks.