Question : What possible steps can you use to debug why outbound SMTP email messages are not reaching intended receipients?

I have 2 clients who have BackupAssist and Backup Exec configured. When the backup job completes, I would like it to email our support email box. But it doesn't. However, the email works for internal (within the clients domains) but not externally. I have even tried to setup email to external emails such as Yahoo.com but no luck.

At this time, what I am looking for is some pointers to find where the email is getting stuck for outbound emails. I am using "Plain SMTP" email message for BackupAssist and I know the SMTP authentication and port 25 is open.

Answer : What possible steps can you use to debug why outbound SMTP email messages are not reaching intended receipients?

http://forums.kayako.com/f84/how-setup-windows-2003-exchange-2003-smtp-relaying-14238/
By default Exchange 2003 is set up too deny relaying from any IP address (including your intranet).
 
So, theoretically, if you set up eSupport on IP 192.168.1.80 and Exchange resides on 192.168.1.25, you need to explicitly tell Exchange that any SMTP connections originating from 192.168.1.80 are allowed.

 8 Simple Steps:
 Log into Exchange 2003 server (192.168.5.25 in my example above) and start Exchange System Manager.      
 Expand the organization_name object, and then expand the Servers node. Expand the server_name object of the server on which you want to control mail relay, and then expand the Protocols node.
 Expand the SMTP node, right-click the virtual SMTP server on which you        want to control mail relay, and then click Properties
Click Relay.
In the Relay Restriction dialog box, several options are available. The Only the list below option is enabled by default; the list below this option is empty.
Click Add. Enter the IP address of the server running eSupport (192.168.5.80 in my example above).
In the Relay Restrictions dialog box, click OK.
Click Apply, and then click OK in the Default SMTP Virtual Server        Properties dialog box.

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For Exchange 2007 : http://www.petri.co.il/authenticated-or-anonymous-smtp-relay-with-exchange-2007.htm

Above are the steps for Exchange 2003 and Exchange 2007 ; depending on which one you are using, check your relay settings and then it will work.