Just to let you all know, I've been working with a user who was having problems with Auto Respond not sending his response message. I added some diagnostic code to try to figure out what the issue was.
He just got back to me, saying that he shortened his message and now it works fine. This leads me to believe that in its current state, Auto Respond may have issues sending long response messages. Usually the limit for each SMS message is 160 characters, so I'm assuming he went above this limit.
I will be looking into this issue, testing and finding the actual limit that causes the issue, and then finding a way to work around this limit. I may have to break the messages up into smaller blocks and send multiple messages, or there may be a way for me to send it that allows for a larger character limit. (I know that above 480 characters, sending as an MMS is usually recommended.)
Once I figure out what this limit is for sure, I may put a character counter under the message telling the user how many characters remain before entering unsafe territory.
Bottom line - I will be working on fixing this, and will hopefully release an update within a couple days to address this issue. I just wanted all of you to be aware that there IS a limit, and if your responses are not being sent correctly you may need to temporarily shorten your response message.