Return Code from sending email using MCFOutboundEmail class

Hi,

I have an App Engine peoplecode that send emails using MCFOutboundEmail class.  After I called the &email.Send(), I checked the return code:

0 %ObEmail_FailedBeforeSending      -- Email failed before being sent.

1 %ObEmail_Delivered                         -- Email was delivered.

2 %ObEmail_NotDelivered                   -- Email was not delivered.

3 %ObEmail_PartiallyDelivered            -- Email has only been partially delivered. Only some of the addresses in the list of addresses were delivered to.

-1 %ObEmail_SentButResultUnknown -- Email was sent but whether it was successful or not is not known.

I searched but couldn't find what would trigger return code -1.  Does anybody have documentation that explains the return code returned by send emails using MCFOutboundEmail class?