OK. I figured it out.
Comcast blocks port 25. I don't know why this is coming up now, unless they've recently started blocking it.
I had to change a line in /etc/default/exim4
From this
SMTPLISTENEROPTIONS='-oX 25 -oP /var/run/exim4/exim.pid'
To this
SMTPLISTENEROPTIONS='-oX 465:25 -oP /var/run/exim4/exim.pid'
I also added this to /etc/exim4/conf.d/main/03_exim4-config_tlsoptions
tls_on_connect_ports=465
It's odd that this just popped up, unless a Debian package updated the /etc/default/exim4 file. It's confusing, but it's working. Hopefully this will be helpful to someone in the future.
Cheers.