The main differences are:

1 - The Registered version faster, because it can use until 200 simultaneous connections to send e-mails, while the Shareware can only use 3 connections;

2 - The Registered version can send limitless e-mails in the same time. The Shareware version only sends the maximum per 100 for time.

3 - The Registered version doesn’t show any type of publicity or message in the sent e-mails. While the Shareware version shows publicity in the page footer of sent e-mails.