OS X V10.7 Lion :: Send Message Content To Server Failed

I have 2 mail accounts setup in Mail. They are on separate domains I host on my server. One account works great, the other gives this error message every time I try to send. "Sending the message content to the server failed." It offeres me to use a different outgoing mail server.I am on latest Lion 10.7.3. mail version os 5.2 (latest). I connect through Airport Extreme.I Googled the problem and searched everywhere. I also tried deleting and setting up the account. I checked all my settings (multiple times). There is nothing to fix in settings. The Connection doctor does not see any problems.�

Here is the really frustrating part: Messages actually get sent. The recipeint always receives them. If I try again with the same server, I get the same error message and again message gets sent. But to Mail it is still unsent and sits in Outbox prompting to send.I also tried the MTU trick. No luck. I should also say that I have seen this problem before on other accounts. I've also seen it come and go. It happened on Snow Leoprad and Leopard too.�