There's still a bug in that class when an email has to be sent to all members of a group.
The addresses are not separated properly with that \n character:

Look at the code:

...
else if (userName.startsWith(MailConstants.PREFIX_GROUP)) {

userName = StringUtils.removeStart(userName, MailConstants.PREFIX_GROUP);

try {

HierarchyManager hm = ContentRepository.getHierarchyManager(ContentRepository.USERS);

Collection users = hm.getRoot().getChildren(ItemType.USER);

Iterator iter = users.iterator();

while(iter.hasNext()){

Content userNode = ((Content) iter.next());

MgnlUserManager manager = new MgnlUserManager();

MgnlUser user = (MgnlUser) manager.getUser(userNode.getName());

if (user.getGroups().contains(userName))

}
}
catch (Exception e)

}
...

the command
ret.append(getUserMail(user.getName()));

just concatinates the adresses. Therefore in the end you have just only one big WRONG email address like, e.g.:
username1@company.comusername2@company.comusername3@company.com

...and finally: it doesn't work!