Hi and welcome to iMore.
Firstly what country do you normally live in and does your carrier allow you to use your phone abroad (data roaming)?
To check if the settings are enabled, go to - Settings > Mobile Data (make sure it's on) > Data Roaming (make sure it's on). Of course, if you do this with the sim from your home country inserted, you will be charged heavily.
Can you find one of the vast number of public wifi spots in the USA and connect to that and try to send an email again?