The compilation serves as a directory correlating email domains to Short Message Service (SMS) gateways offered by mobile network operators. This mapping enables users to transmit electronic mail messages directly to cellular phones as text messages. For example, a message sent to `1234567890@txt.att.net` would deliver a text to the AT&T subscriber with the phone number 123-456-7890.
This type of directory facilitates communication with individuals who may not have consistent internet access or prefer receiving notifications via SMS. It has been instrumental in time-sensitive alerts, emergency notifications, and appointment reminders. Its historical significance lies in bridging the gap between the internet-based email system and the mobile phone network, especially during periods of limited smartphone adoption.