Be natural, this can be annoying if you're apologizing too often, [write your message in plain language—like a human talking to another human][1]. If error is caused by you (your app, your server e.t.c.) add apology, otherwise leave just statement of fact and how to resolve the issue. 

For example:

- "Sorry, we couldn't send your message because of problem on our server. Please try again."
- "We couldn't send your message, please fill email field."

Another one:

- "Sorry, you don't have permission to access this feature anymore, your balance was low and we downgraded your plan."
- "You don't have access to this feature, it's available only in our Extended plan."


  [1]: http://www.uxmatters.com/mt/archives/2010/08/avoid-being-embarrassed-by-your-error-messages.php