Oftentimes it is very helpful to remove your mail server from the equation to see if there is an underlying network / reputation problem blocking mail flow. Here's how you can manually send a test message using the telnet command built into every operating system. You'll need to determine the name or address of your recipeints mail server. You can do this by looking up their MX record at http://mxtoolbox.com. In my example the MX record is a1.mx-route.com. Your commands are in bold below and the responses I got are in italic. Be careful, on Windows if you typo you cannot backspace and correct yourself. Just hit enter and retype the entire command.
telnet a1.mx-route.com 25
Trying 208.123.79.41...
Connected to a1.mx-route.com (208.123.79.41).
Escape character is '^]'.
220 a1-1.mx-route.com ESMTP
helo mxtoolbox.com
250 a1-1.mx-route.com
mail from: <peter@mxtoolbox.com>
250 sender <peter@mxtoolbox.com> ok
rcpt to: <support@mxtoolbox.com>
250 recipient <support@mxtoolbox.com> ok
data
354 go ahead
Subject: Test Message
This is a test message.
.
250 ok: Message 156715331 accepted
quit
221 a1-1.mx-route.com
Connection closed by foreign host.
Here you can see that the sender, recipient and message were accepted by the 250 responses from the recieving mail server. If there are problems you will see them reflected with 4xx or 5xx responses that can be very helpful for figuring out the problem.
Update: As of Feb, 5th 2009 the MxToolBox Forums have moved to http://community.mxtoolbox.com/forums
Peter
Product Development Engineer
peter@MXToolBox.com
512-637-8762