While working to email-enable SharePoint lists I ran across a great post on Microsoft Support about testing SMTP:

SMTP relay behavior in Windows 2000, Windows XP, and Exchange Server

The basic test:

Start a command prompt.

Type telnet ServerName 25, where ServerName is the SMTP server name or IP address and 25 is the port number, and then press ENTER.

Type EHLO, and then press [ENTER].

At the Telnet session prompt, type RSET. The telnet session responds with text that is similar to the following:

1. 250 2.0.0 Resetting

2. Type MAIL FROM:UserName@DomainName.tld, where UserName is the name of the user, DomainName is the name of the domain, and tld is the top level domain such as .com or .net.
   The telnet session responds with text that is similar to the following:

3. 250 2.1.0 UserName@DomainName.tld….Sender OK

4. Type RCPT TO:RecipientName@DomainName.tld, where RecipientName is the e-mail address of the recipient.
   The telnet session responds with text that is similar to the following:

5. 550 5.7.1 Unable to relay for RecipientName@ DomainName.tld