Question About mail send!..

[erduscuk]

Hi
I have a physical server.
Installed with Plesk control panel.
There are 14 hosting.
Domains in the server can send emails to them.
I can get an email from everywhere. (Other IP and domain names, gmail, hotmail, yahoo etc.)
But I can not send emails to other domains.

Sample :
My Server
--- a.com: Can receive emails from b.com and c.com. Only b.com can send an email.
---b.com: can receive email from a.com and c.com. Only a.com can send an email.

Other Server
-c.com: a.com and b.com can send an email. But it does not get email from a.com and b.com.