TCP port "smtp" is not defined for remote smtp transport

Problem: Exim error log (/var/log/exim_mainlog) shows the following error while sending mail,

"TCP port "smtp" is not defined for remote_smtp transport"

Solution:
The file /etc/services is the cPanel file which specifies all the services running on the server along with the
assigned port numbers.
For smtp it should contain the following lines,

root@acela [~]# grep smtp /etc/services
smtp            25/tcp          mail
smtp            25/udp          mail
smtps           465/tcp                         # SMTP over SSL (TLS)

Check the permission of the file /etc/services.
It should be 644.

root@ac [~]# chmod 644 /etc/services
root@ac [~]#

This will fix the mail error.