This is a simple package allowing you to send already composed MailMessages through your registered default email client on Windows (such as Outlook or Eudora). To invoke it, merely ask it to
MAPIClient sendMailMessage: aMailMessage.
and it will take you from there. On Squeak versions which have the AppRegistry installed (which is included in 3.6 and later versions) MAPIClient will register itself as a MailSender which means you can use it to do any kind of mailing through your default email client (including bug-reports, the bug fix archive viewer etc.)