But two days ago Comcast finally started blocking port 25 on me (http://customer.comcast.com/help-and-support/internet/email-client-programs-with-xfinity-email/).
There are quite a few posts and articles about switching to port 587. Below is what I found worked on my Fedora 20 server.
Step 1. New /etc/mail/sendmail.mc config setup:
define(`RELAY_MAILER_ARGS', `TCP $h 587')dnl
define(`ESMTP_MAILER_ARGS', `TCP $h 587')dnl
Step 2. Setup authinfo
Assuming your comcast account is firstname.lastname@example.org with password door123 ...
# touch /etc/mail/authinfo && chmod 600 /etc/mail/authinfo
# echo 'AuthInfo:smtp.comcast.net "U:walrushose" "P:door123" "M:PLAIN"' > /etc/mail/authinfo
# makemap hash /etc/mail/authinfo < /etc/mail/authinfo
# chmod 600 /etc/mail/authinfo.db
Step 3. Remake and restart sendmail
# make -C /etc/mail
# systemctl restart sendmail.service
Now my server is happily using port 587 for outgoing email.