What are your SMTP settings? SSL? TLS? Password? Port?

The first thing I'd try is connecting to the SMTP server by hand. Telnet (using your favorite telnet application, or, if you don't have one, telnet.exe from the command prompt) to your SMTP server on port 25 ("telnet.exe smtp.example.com 25"). Then try to send an email using this script:

Quote:
220 smtp.example.com status
HELO workstation.example.com
250 smtp.example.com Hello
MAIL FROM:<email@example.com>
250 2.1.0 email@example.com....Sender OK
RCPT TO:<email@example.com>
250 2.1.5 email@example.com
DATA
354 Start mail input; end with <CRLF>.<CRLF>
From: <email@example.com>
To: <email@example.com>
Subject: testing email server

this is the body of the test
.
250 2.6.0 <blahblah> Queued mail for delivery
QUIT
221 2.0.0 smtp.example.com Service closing transmission channel

Red text indicates server responses, which will be similar to, but not exactly what I listed. Obviously, change out hostnames and email addresses for accurate data.