Je to obdobné jako v html (http://www.jakpsatweb.cz/mail.html)

var em_subject, em_body, em_mail : string;
begin
  em_subject := 'This is the subject line';
  em_body := 'Message body text goes here';

  em_mail := 'mailto:delphi.guide@about.com?subject=' +
    em_subject + '&body=' + em_body ;

  ShellExecute(Handle,'open',
    PChar(em_mail), nil, nil, SW_SHOWNORMAL) ;
end;