As of last night, every iMessage I send is showing "Not Delivered." This only happens with one contact, and he is still receiving the messages I've sent, despite their apparently undelivered status. The same is happening to him, with me - his phone is showing "Not Delivered," but I am receiving all his messages (and again, it only does this for me on his phone).

Another issue I'm having - but less frequently - is iMessages disappearing. I will send them, it will show up in the app as sent/delivered, but when I check back a few hours later, the message has completely disappeared and the recipient has not received them.

Any idea what the issue could be here? I'm running iOS 5.1.1 on an iPhone 4S. It's not a big deal, I suppose, but I find it slightly odd.