First, please update to the latest development build (from this forum). It may not resolve the issue, but it has better error reporting.
Second, one or two other users reported this to me before, and it turned out to be a server thing.
In particular, when your mail account has a "canonical" notation (e.g. john.smith@corp.com) and a shorthand (e.g. js@corp.com), sometimes the server is configured to only allow sending from one but not the other.
This is a server policy thing.
Both stock Email and Enhanced Email use a different mail protocol (ActiveSync). A comparison with Mail Droid would be more appropriate (also EWS).