Send an email using the command line
Ever get an email failure without a good error message and want to see what's really going on? Here's how to send an email using nothing but telnet. You'll see exactly where the process breaks down and point you towards the real problem.
telnet mail.domain-you-are-sending-mail-to.com 25
helo domain-you-are-sending-from.com
mail From: user@domain-you-are-sending-from.com
rcpt To: user@domain-you-are-sending-mail-to.com
data
From: user@domain-you-are-sending-from.com
To: user@domain-you-are-sending-mail-to.com
Subject: Testing
(Newline)
This is just a test message 
(Newline)
.
(Newline)
That's it. If all goes well you'll see "250 OK" with some variables afterwards. This means the server accepted the email and should deliver it shortly. If you get an error then the number code will point you in the right direction.
For more information on SMTP error codes enjoy the lovely RFC