Major U.S. Providers:

AT&T
SMS: <10-digit-number>@txt.att.net
MMS: <10-digit-number>@mms.att.net

Verizon
SMS: <10-digit-number>@vtext.com
MMS: <10-digit-number>@vzwpix.com

T-Mobile
SMS: <10-digit-number>@tmomail.net
MMS: <10-digit-number>@tmomail.net

Sprint (now merged with T-Mobile, but some legacy users may still use Sprint domains)
SMS: <10-digit-number>@messaging.sprintpcs.com
MMS: <10-digit-number>@pm.sprint.com

Boost Mobile
SMS: <10-digit-number>@sms.myboostmobile.com
MMS: <10-digit-number>@myboostmobile.com

U.S. Cellular
SMS: <10-digit-number>@email.uscc.net
MMS: <10-digit-number>@mms.uscc.net

Metro by T-Mobile (formerly MetroPCS)
SMS: <10-digit-number>@mymetropcs.com
MMS: <10-digit-number>@mymetropcs.com

Cricket Wireless
SMS: <10-digit-number>@sms.cricketwireless.net
MMS: <10-digit-number>@mms.cricketwireless.net

Google Fi
SMS/MMS: <10-digit-number>@msg.fi.google.com

Virgin Mobile USA
SMS: <10-digit-number>@vmobl.com
MMS: <10-digit-number>@vmpix.com



Carrier	            SMS Gateway	                        MMS Gateway
AT&T	            number@txt.att.net	                number@mms.att.net
Verizon	            number@vtext.com	                number@vzwpix.com
T-Mobile	        number@tmomail.net	                number@tmomail.net
Sprint	            number@messaging.sprintpcs.com	    number@pm.sprint.com
Boost Mobile	    number@sms.myboostmobile.com	    number@myboostmobile.com
Cricket	            number@sms.cricketwireless.net	    number@mms.cricketwireless.net
US Cellular	        number@email.uscc.net	            number@mms.uscc.net
Google Fi	        number@msg.fi.google.com	        number@msg.fi.google.com
Virgin Mobile       number@vmobl.com                    number@vmpix.com
Metro by T-Mobile   number@mymetropcs.com               number@mymetropcs.com