can't send mail with webmail outside the server

[oliviercreacom]

I'm using Plesk 7.6 for Windows on a 2003 server

For any of my domains on this server, i can't send mail with webmail outside if the recipient is outside the domain. But it work fine with other mail program like outlook or thunderbird.
For instance on this server i have mydomain1.com and mydomain2.com, and on another server i have outsidedomain.com
With webmail.mydomain1.com i CAN send mails to mydomain2.com but not to outsidedomain.com, the same for webmail.mydomain2.com and any other domains from my server.
But i can send mail from mydomain1.com to outsidedomain.com with Thunderbird or Outlook ?

Have you heard of that ? What could be the cause ?
My client need to have the use of webmail because they're having trouble with their computer connection at work ATM (so they can''t use Outlook)

Thank you