Hi

The below will help you.
1) Check the mail.log file to see what type of error you are receiving?
2) It could be possible that at the receiver's end they will block the email?
3) The reason also could be that the receiver's email box is full?
4) It could be possible that you can send mail to your company's email id. Not
sure how this is configured. I am experiencing this problem and I posted the
ticket here for help.

Hope the above helps.

Thanks,
Satheesh.