An SMS (short messaging service) email extension is a line of text that you must add to the ending of a cell-phone owner's number if you want to send him a text message via email. There are four major cell-phone providers in the United States, and all of them have a unique code that allows you to address short messages to their various cell-phone customers. Keep in mind when sending your emails that you should keep your message very short.
The email extension for Verizon Wireless users is "@vtext.com"—add this extension after the full cell-phone number. When you send a text message to a Verizon customer you have a 160-character limit per text—any email you send longer than that will separate into two or more messages.
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You can reach an AT&T Wireless cell-phone account holder by emailing the phone number followed by "txt.att.net." Like Verizon, AT&T also has a 160 character limit on each text message. Keep in mind that AT&T Wireless now includes former customers from Alltel and Cingular.
Sprint, which merged with Nextel in 2005, has two different extensions for subscribers. Before you attempt to send your message, find out if the person you want to text is a former Nextel customer first. Use "@messaging.sprintpcs.com" to send a text message via email to a Sprint user and "@messaging.nextel.com" if it's a former Nextel customer. In either case, the message size limit is 500 characters for Sprint (and Nextel) users.
T-Mobile, another popular mobile service, also allows you to send short messages via email. If you need to send your text message via T-Mobile, type "@tmomail.net" after the full cell-phone number. T-Mobile permits 160 characters per text.