Before you do anything, just check, if you do not (or do, if desired) SmartHost all mail to your ISP's mail server. Check in MailEnable under Properties of SMTP Connector in Smart Host tab. If you have SmartHosting enabled, maybe your smarthost server does not allow you to relay.
If this is not the case, then you may try manually connecting from this server (which you installed MailEnable on) to some exact recipient via TELNET.
So, open Comand prompt (run "cmd") and then type in "telnet" and you are in Telnet prompt. Then:
1.) First, try to connect to your own MailEnable server:
o mail.yourMachineDomain.com 25
Your MailEnable server should respond with 220 code and Welcome message. If this is the case, you may type "quit", since your mail server responds.
2.) then try connecting to your ISP (or any other known) mail server:
o mail.yourISPprovider.com 25
Same as above, your ISP's mail server should respond with 220 code and Welcome message. If this is OK, then you may proceed sending mail out to the problematic mail address, by continuing with the following commands:
HELO yourname
MAIL FROM: your.legitimate@email.address
RCPT TO: problematic@recipient.com
DATA: type some mail content
when finished, press ENTER, then dot "." and ENTER again
finish with QUIT
If everything is properly opened and connection not blocked, you should connunicate with recipient's mail server without problems. But if you still get an error, report back with error.