E2k + Outlook…can’t send mail

I created accounts for our users located in remote area on E2k.I cofigured their outlook to send and receive mail using internet mail service.Then I tested couple accounts and I was able to recieve messages from inside and outside our organization.However, I can not send messages
to accounts outside the organization. When ever I try to send mail to outside of our org I get the following error messages “Your message did not reach some or all of the intended recipients.” + ” No transport provider was available for delivery to this recipient.”

I appreciate any kind of help!