Each phone carrier has its own hostname. To send a text message via email, you’ll need to use a SMS or MMS to SMTP Outgoing email gateway. Just substitute a 10-digit cell number for ‘number’ for each carrier below:

AT&T: number@txt.att.net (SMS), number@mms.att.net (MMS)
T-Mobile: number@tmomail.net (SMS & MMS)
Verizon: number@vtext.com (SMS), number@vzwpix.com (MMS)
Sprint: number@messaging.sprintpcs.com (SMS), number@pm.sprint.com (MMS)
XFinity Mobile: number@vtext.com (SMS), number@mypixmessages.com (MMS)
Virgin Mobile: number@vmobl.com (SMS), number@vmpix.com (MMS)
Tracfone: number@mmst5.tracfone.com (MMS)
Metro PCS: number@mymetropcs.com (SMS & MMS)
Boost Mobile: number@sms.myboostmobile.com (SMS), number@myboostmobile.com (MMS)
Cricket: number@sms.cricketwireless.net (SMS), number@mms.cricketwireless.net (MMS)
Republic Wireless: number@text.republicwireless.com (SMS)
Google Fi (Project Fi): number@msg.fi.google.com (SMS & MMS)
U.S. Cellular: number@email.uscc.net (SMS), number@mms.uscc.net (MMS)
Ting: number@message.ting.com
Consumer Cellular: number@mailmymobile.net
C-Spire: number@cspire1.com
Page Plus: number@vtext.com