I have been receiving large number of sms.ac invitations in the past months. I have never responded to those invitations, but they are very annoying and it seems like my friends are spamming me with those invitations.
Yesterday, it drove me nuts. The invitations are repeating every few days, warning me that I am not responding to requests and sms.ac keeps on resending the requests!
Anyways, [a simple google search|http://www.google.com/search?ie=UTF-8&oe=UTF-8&q=sms%2Eac] shows that sms.ac does both; scam and spam.
Now, to google before every web service they join is too much trouble for my ''Fwd:Fwd:friends''.
How can I tell them to be a little bit more cautious?
__UPDATE:__ Ok, solution is very simple. Create a filter rule in your mail application to move any mail that matches invitiation.sms.ac (in the from: field) to Trash.