I have just contacted iMail and they have informed me that this is a known issue on their side. The raw error message you are seeing is originating on their servers and getting posted back to us over the API.
I’m very sorry about this, I’m disappointed as we have had a few intermittent issues with iMail going back to mid November, these are mostly related to their server migration. Currently they do not know the full scope of the problem but they have people looking into this right now.
I will update you once further information comes through to me.
I have since received more information on this problem from iMail. The original issue you reported started on Wednesday 16th and was fully resolved on Monday 21st at around 18:00.
The mail flagged as “Object reference not set to an instance of an object” was actually reaching their queuing system and would have been dispatched. It’s just they were sending back this corrupted response message which triggered our system to mark as “FAILED”.
You will not be charged for any mails affected by this problem. I have been monitoring the iMail service this week and everything does now seem to be getting processed in the normal way.