Specifies the prefix used in email addresses.
Carrier information is used when the device tracker gateway needs an SMS email address to be generated from an automatically-tracked device address. Addresses are generated in the following form:
email-address = sms_email_user_prefixphone-number@sms_email_domain
sms_email_user_prefix is the value of the sms_email_user_prefix property
the phone number comes from the ml_device_address.address column
sms_email_domain is the value of the sms_email_domain property