For about 2 weeks now I have been having issues intermittently where I get a red exclamation mark and it states message not delivered. This is when it is running via iMessage, the strange thing is, it doesn't send as a text either, and it won't resend a few seconds later either, I have to manually resend the message. It's only happened maybe 6 times over the last few weeks, but had never occurred before.
In fact today I sent a few messages and didn't hear a response, I then picked up the phone and called to find out what I had sent the text for. When my call connected the text message went through to the person I was contacting. Hopefully this is just a temporary glitch and gets better before it gets worse, nothing worse than unreliable text messages...
I know there is a separate issue with iMessage when you are signed in on multiple devices, it doesn't always send the message to both devices. I also use my email addy as my iMessage account and when I have sent an email from my iPhone it did not get delivered to my email account, however it went through iMessage instead. For that issue I think I may make a separate @me account to solve that little issue, as my intention was for the message to go to my email account.