The Simple Mail Transfer Protocol (SMTP) is the service responsible for transmission of electronic mail. The service by default runs on port 25. It is useful to enumerate the SMTP service in order to know the server version along with the command it accepts. We can use the Nmap syntax, as follows, to enumerate the SMTP service:

nmap -p 25 -T4 -A -v <Target IP address>

The following screenshot shows the output of the enumeration command we fired. It tells us that the SMTP server is of type Postfix and also gives us the list of commands it is accepting: