Sendmail [25: Connection refused] fix
If you can't receive email from the outside and got next error:
(Delivery Status Notification [(0) www.domainname.com. [18.104.22.168]:25: Connection refused]).
You should check the iptables , is port listening .
if you can telnet on port 25 from localhost and can't from remote you need to modify your sendmail.cf file. Just comment the next line:
# O DaemonPortOptions=Port=smtp,Addr=127.0.0.1, Name=MTA
Save config and restart sendmail.