MigrateMailIgnore is only used when there are custom mail systems present.

In a default drupal installation the MigrateMailIgnore class is not used and email will be sent out anyway.

If there are no custom modifications to the mail system then $conf['mail_system'] is empty. DefaultMailSystem is provided as default during variable_get().
The code in migrate that is supposed to stop emails from being sent (implemented in #1254398: Mail system class to prevent email during migration and documented at http://drupal.org/node/1555278) does not do anything when $conf['mail_system'] is empty.

A workaround would be to edit settings.php and add a $conf['mail_system']['default-system'] = 'DefaultMailSystem'; so that migrate will override it.

A fix is provided in attached patch will also disable email when no mail_system modifications have been made.