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.

Sendmail: Prvent your SMTP servers from being blacklisted or graylisted

Sometimes we need to send email updates to millions of customers, the customer may register using their office email or public email like yahoo.
If you send a huge amount of emails in a short period of time, you may risk your SMTP servers beging blacklisted or graylisted.

There are a few ways to ensure your email server's functionality.

1. Use sendmail's greet_pause
In sendmail.mc, define greet_pause as 500 mili seconds.

FEATURE(`greet_pause',500) 

update sendmail.cf:

m4 sendmail.mc > sendmail.cf

restart sendmail service:

service sendmail restart

By defining greet_pause as 500, you are telling your email server to pause for 500 mili seconds before responding to any EHLO request. So it can control the rate you send email to outside world, preventing you from flooding outside email servers.

2. rotate IP addresses of your SMTP server periodically.
Suppose you are assigning management IP to eth0, you can define eth1 as the default outgoing IP address, 192.168.100.101, 192.168.100.102, 192.168.100.103 are reserved for eth1, you can write script as cron to rotate them every day.

But you have to make sure these two things:

• These 3 IP addresses are translated to 3 different public IPs on your router or firewall.
• The 3 IPs are legitimate addresses to send out email for domain in your header FROM, you need to define you dns records and reverse resolution properly. Otherwise your mails may be rejected by other servers.

Note: A few years ago, I manged 20+ email servers as part of my work, every day we need to send out tons of emails. Customers register to receive email updates from us, I am not a spammer :)