Use telnet to test SMTP server

Besides testing HTTP server, telnet can also be used to test SMTP server

below is the testing I did on my linux box

$ telnet localhost 25

220 redhat.localdomain ESMTP Postfix

ehlo abc.com

250-redhat.localdomain

250-PIPELINING

250-SIZE 10240000

250-VRFY

250-ETRN

250-ENHANCEDSTATUSCODES

250-8BITMIME

250 DSN

mail from: scripter@linux.com

250 2.1.0 OK

rcpt to: nonexsit

550 5.1.1 <nonexist>: Recipient address rejected: User unknown in local recipient table

rcpt to:tom

250 2.1.5 OK

data

354 End data with <CR><LF>.<CR><LF>

Subject: hi this is subject

this is email body

more chars

.

250 2.0.0 OK: queued as 25AE12AD1

quit

221 2.0.0 Bye

in /var/spool/mail/tom will see the email we just sent

please note the lines starting with numbers are the output returned by SMTP server, the numbers are the status code, search "smtp status code" for more details.
"ehlo", "mail from:", "rcpt to:", "data", "quit" are the commands sent to SMTP server.