I'm sure you're pretty attached to Incredimail, but it's really an incredibly poorly designed product (from the backend). It violates numerous RFCs and the behavior you are experiencing is likely a result of that.
Has this always happened? What mail server are you using? Your mail server is expecting the message to come through in a specific format, which Incredimail is not sending it in.
Mike
Whenever I try to send an email thru Incredimail the message "501 space seperator provided without argument" comes up.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic