Outlook 2010 - Mail will not send when "Send" is clicked.

I don't use Outlook so I am unable to figure this one out.

My son uses Outlook 2010 and is having to go to some extra steps every time he wants to send a mail. Here's what is happening:

• He composes or replies to a mail and clicks "Send".
• It goes to Out Box and is not sent.
• It sits in the outbox indefinitely, even for days as other mail is collected automatically every two minutes. He says he has Outlook 2010 configured to send on receive/check; but, it does not send.
• In order to send that mail, he must open the Out Box and then open that mail (that is just sitting there unsent) and click "Send" again and then and only then does it get sent -- from Out Box.

This happens with all mail he sends.

Suggestions?