Changing from HTTP to HTTPS results in ERR_CONNECTION_REFUSED for both Mattermost & GitLab

Okay, I've definitely confirmed its something with the mail settings, but I can't figure out what. Everything looks perfectly correct to me, and using the exact same settings via system console sends mail just fine. But when I try to configure it in gitlab.rb it crashes mattermost.

Here's what I'm using in gitlab.rb, this all looks good, right? The names for each mattermost rule are correct?

## SMTP Settings
mattermost['email_feedback_name'] = "GitLab Mattermost",
mattermost['email_feedback_email'] = "git@mydomain.com", # removed for security
mattermost['email_smtp_username'] = "git@mydomain.com",  # removed for security
mattermost['email_smtp_password'] = "************",      # removed for security
mattermost['email_smtp_server'] = "mail.mydomain.com"    # removed for security
mattermost['email_smtp_port'] = "26"
mattermost['email_connection_security'] = ""

Here's what my cPanel says I need to use for email settings:

# Username:        git@mydomain.com
# Password:	   Use your cPanel password.
# Incoming Server: mail.mydomain.com
# IMAP Port:       143
# POP3 Port:       110
# Outgoing Server: mail.mydomain.com
# SMTP Port:       26
# Authentication is required for IMAP, POP3, and SMTP.

No idea what I'm doing wrong. Password is correct, email is correct, port is correct, domains are correct... Very strange.