eM client can receive but cannot send email internally

We have setup the eM client 6.0.19861.0 trial version with the company MS Exchange. It is not expired yet. All the old email are being synched. The client can receive email from both internal and external email addresses, but it cannot send any mail internally. It came back as failure to deliver message. Is there anyway to fix this? Any advice is appreciated.

Hi, could you provide screenshot of your error message?