Intermittently Validation Failed for single user

[nh-mike]

Hi,

I have been running Z-Push for some time now and have been finding than only one of my users reports that their phone won't load email in the morning. Upon looking at my logs, I am presented with line upon line of errors for that mailbox which read as such:
[ERROR] [mailbox] z_RFC822 error: Validation failed for: <.>
I scanned over the code in z_RFC822.php and found that this is related to failed validation of a mailbox.
Would I be correct to understand that this would be a bad request on part of the device?
The device in question is an iPhone 6 running the current version of IOS and we are using the Apple Mail app.