Emails from hosted domains not routing to 3rd party servers [Archive] - Parallels Forums
"enable mail management functions in panel" is the wrong setting.
Domains -> <DOMAIN> -> Mail -> Change Settings -> "Activate mail service on domain" is what you search...
The solution might be simpler than what has been suggested here. To send mail generated from abc.com (that is running on your Plesk server) to the mail server (abc.com) that is running on a different mail server, the easiest approach is to keep the "enable mail management functions in panel" checked (somewhat counterintuitive) but then you need to take the extra step of accessing the domain panel as danliker suggested (Domains -> <DOMAIN> -> Mail -> Change Settings ->) but UNCHECK "Activate mail service on domain"
This effectively enables PostFix to rout mail that is destined to "firstname.lastname@example.org" correctly to your external mail server.
Please post your results if this works (or doesn't)!