I have no idea if this might be your problem but we get all of our email sent to our gmail address - every now and then gmail suddenly gets a bit over-sensitive in the spam detection and puts order confirmation emails and messages from our enquiry form into the spam folder.

If I see an order in admin that has not come through via email - the first place I look is in the spam folder - 99% of the time, it is there. After a day or 2, gmail then goes back to normal and messages are sent where they are supposed to be.

I have put all of our emails into the trusted white list but it makes no difference - cannot work out why gmail does this occasionally and why/how it gets over it and behaves normally again. I dont worry about it now as these messages are easily found when needed (I automatically star all zencart/admin messages so I can pick them out quickly).

cheers
Leah