Okay, so after var_dumping literally everything, I learned.
cPanel's local mail server randomly made a rule that redirects all SMTP (insecure or secure) to 127.0.0.1 w/ iptables
An exception is raised but is suppressed by a try/catch and doesn't tell the admin what's happening.
My problem is...
Randomly encountered a problem on the latest XenForo version.
https://xenforo.com/community/threads/test-outbound-email-broken-smtp-mail-option-not-working-sending-emails.221318/
Previous references to the same problem do exist (linked above) but should've been fixed as of XF 2.3.0 Release...