To send sms freely, you have know the operator and the area of the service. Then, open your webmail like Gmail, Yahoo, or Rediff and send email to the MobileNo@OperatorWebsite.com and the mobile user receives it as a normal SMS, but "from" will show your email address rather than mobile number. I use Aritel AP and the messages are delivered instantly like the normal SMS. If you have to send message to an Airtel Mobile in AP just send email to MobileNo@airtelap.com.

Andhra Pradesh AirTel-----------Mobile No@airtelap.com
Andhra Pradesh Idea Cellular-----Mobile No@ideacellular.net
Chennai Skycell/Airtel------------Mobile No@airtelchennai.com
Chennai RPG Cellular-------------Mobile No@rpgmail.net
Delhi Airtel-----------------------Mobile No@airtelmail.com
Delhi Hutch-----------------------Mobile No@delhi.hutch.co.in
Gujarat Airtel---------------------Mobile No@airtelmail.com
Gujarat Idea Cellular--------------Mobile No@ideacellular.net
Gujarat Celforce/Fascel-----------Mobile No@celforce.com
Goa Airtel-------------------------Mobile No@airtelmail.com
Goa BPL Mobile-------------------Mobile No@bplmobile.com
Goa Idea Cellular------------------Mobile No@ideacellular.net
Haryana Airtel--------------------Mobile No@airtelmail.com
Haryana Escotel-------------------Mobile No@escotelmobile.com
Himachal Pradesh Airtel-----------Mobile No@airtelmail.com
Karnataka Airtel-------------------Mobile No@airtelkk.com
Kerala Airtel-----------------------Mobile No@airtelkerala.com
Kerala Escotel---------------------Mobile No@escotelmobile.com
Kerala BPL Mobile-----------------Mobile No@bplmobile.com
Kolkata Airtel----------------------Mobile No @airtelkol.com
Madhya Pradesh Airtel-------------Mobile No@airtelmail.com
Maharashtra Airtel-----------------Mobile No@airtelmail.com
Maharashtra BPL Mobile-----------Mobile No@bplmobile.com
Maharashtra Idea Cellular----------Mobile No@ideacellular.net
Mumbai Airtel----------------------Mobile No@airtelmail.com
Mumbai BPL Mobile----------------Mobile No@bplmobile.com
Punjab Airtel-----------------------Mobile No@airtelmail.com
Pondicherry BPL Mobile------------Mobile No @bplmobile.com
Tamil Nadu Airtel-------------------Mobile No@airtelmail.com
Tamil Nadu BPL Mobile-------------Mobile No@bplmobile.com
Tamil Nadu Aircel-------------------Mobile No@airsms.com
UP (West) Escotel-------------------Mobile No@escotelmobile.com

Madhya Pradesh Airtel 9893 919893xxxxxx@airtelmail.com
Maharashtra Airtel 9890 919890xxxxxx@airtelmail.com
Maharashtra BPL Mobile 9823 9823xxxxxx@bplmobile.com
Maharashtra Idea Cellular 9822 9822xxxxxx@ideacellular.net
Mumbai Airtel 9892 919892xxxxxx@airtelmail.com
Mumbai BPL Mobile 9821 9821xxxxxx@bplmobile.com
Punjab Airtel 9815 919815xxxxxx@airtelmail.com
Pondicherry BPL Mobile 9843 9843xxxxxx@bplmobile.com
Tamil Nadu Airtel 9894 919894xxxxxx@airtelmail.com
Tamil Nadu BPL Mobile 9843 919843xxxxxx@bplmobile.com
Tamil Nadu Aircel 9842 9842xxxxxx@airsms.com
Uttar Pradesh (West) Escotel 9837 9837xxxxxx@escotelmobile.com

Chennai Hutch 9884 9884012345@south.hutch.co.in
Andhra Pradesh Hutch 9885 9885012345@south.hutch.co.in
Karnataka Hutch 9886 9886012345@south.hutch.co.in
Karnataka Airtel phonenumber@airtelkk.com

MSC Code for Cellular Circle Operations
Supported Networks

Airtel: Andhra Pradesh, Madhya Pradesh, Chennai (Skycell), Delhi, Gujarat, Haryana, Himachal Pradesh, Goa, Karnataka, Kerala, Kolkata, Maharashtra, Mumbai, Punjab, Tamil Nadu, Uttar Pradesh (West)
Hutch: Delhi
Escotel: Haryana, Uttar Pradesh (West), Kerala
BPL Mobile: Mumbai, Maharashtra, Kerala, Tamil Nadu, Pondicherry, Goa
Idea Cellular: Andhra Pradesh (Tata), Maharashtra, Goa, Gujarat
RPG Cellular: Chennai
Aircel: Tamil Nadu
Celforce (Fascel): Gujarat

BSNL: You can send SMS emails to BSNL phones as mobilenumber@bsnlumN.com. The email domain is different for each region - bsnlumN.com (North), bsnlumS.com (South), bsnlumE.com (East) and bsnlumW.com (West).

Note: When I checked for the DNS records of airtelmail.com, I have seen that it has been owned by a company in Florida, USA and not by Bharthi, so I am in doubt and don't think it works. Rest are okay. Check it and post your comments.