UniMail 1.01e - is a program for your Newton MessagePad that can connect to a UNIX host via modem or serial line and send or receive Internet mail.  It also provides a nice

interface for reading, sending and organising your mail. FEATURES Simple login scripting for connecting to your internet service provider. Clear and easy to use interface. Smart text wrapping to overcome the Newton's small screen width. Full threading of email items with the same subjects. Runs at up to 57,600 bps with MNP error correction. Works with mobile phones with the appropriate hardware connectors

In short it's the most functional email system for handheld computers today.