smtp error in delivering
 

I compose a message, (can be reply or forward). When I click the send/receive
button, the messages in the outbox will not send. I checked log files in mail
server.
SMTP protocols start but at the end my outlook don't send "." command and
server give that message on log :
"Transmission did not end with "." on a line by itself. Disconnecting..."
Then I delete the message in outbox. Sometimes I have to restart program or
computer to send a new mail. What is the problem and
What can I do to solve this problem.