[otrs] Cannot send email through OTRS.

Hi !!! I have installed OTRS and it works fine with one exception send email. If i send to my internal mail it works but if is one outbound email this just stay in somewhere I couldn't find and after five days I get a message that wasn't deliveried. I have _ Linux Fedora Core 5 2.6.17-1.2157 - Sendmail 8.13.1 - CyrusImap V2 The system works fine but I cannot send message to my customers. Does anyone can help me? Thanks Daniela