text messages beginning with '!' trigger automated responses? Whenever I send a text message to my friend with an Android phone (further testing has shown this happens with iPhones as well, though not blackberries) that starts with '!', I get an automated response. I've detailed it in full, but to summarize:

Me sending an SMS to my friend:
"!test"

results in me receiving an immediate response:
"message test to XXXXXXXXXXX succeeded"

and my friend receives:
"test"

XXX is where the destination phone number is. I find it concerning because I suspect it is the phone itself that is sending this automated reply, and for anyone who has to pay for outgoing text messages this could be an avenue for a third party to inflate their bill without them even knowing.

Does anyone know what this is, why it's happening, or how to disable it?
That appears to be your carrier's delivery report code. Many (but not necessarily all) carriers implement it in some form or another, but the exact method can vary pretty widely. It's briefly mentioned on the SMS Wikipedia article.

I don't recall exactly when the network sends the delivery report, but I'm fairly positive that it wouldn't be originating from the phone, and therefore wouldn't count against any SMS limit it may have. The device may respond to the provider when it receives the message (though it may not since the carrier may send the report at some other point in the process), but the actual SMS you get back should be originating from the network's SMS gateway, I believe. The phone's OS (or SIM) may have some bearing on whether or not it "honors" the request, but I'm not sure.

I found a (probably incomplete) list of some of the various carrier codes here:

UK
T Mobile: *0#
O2: *0#
Orange: RCT followed by a space
Virgin: *0#

US
T-Mobile: *noti#

Germany
T-Mobile: *T#
Vodafone D2: *N#
O2: *N#
e-plus: *N#

Sweden
Telia: *KV#
Comviq: KV#
Telenor: ##

Brasil
Claro: *N#

Switzerland
SWISSCOM: *N#

Turkey
TURKCELL: *N#

Poland
PlusGSM (Polkomtel): XYZ

Croatia
T-Mobile: *r*

Portugal
Vodafone: *NOT#

Hungary
T-Mobile: *A#

India
Voodafone: *NOT#

Qatar
Qtel: *N#

Belgium
Proximus: *R*
Base: *N#

Greece
Vodafone: *#
Wind: *0#

Serbia
MTS: *!#
Telenor: *!#

Romania
Orange: *NM#

Israel
Orange: *NM#