Outgoing mail not working... MX records?

[NeuroPsyche]

Since I installed Plesk 11.x our servers can't send mail to an external email server domain.

The recipient email server responds back that it cannot accept the mail.

The issue doens't seem to be related to Plesk at all and should be MX record routing at the registrar.

Does anyone have a template or example of a registrar entry for MailEnable?
For the MX entry @ registrar what would be the subdomain, if any?
ex: MX pop.<domainname>.com
ex: MX smtp.<domainname>.com
???

Greatly apprecaite anyone's help.