Have you sent an email to a group to find that some people from the group didn't receive it and some people who are no longer in the group did receive it? This is because the old version of the group has been cached (saved) on your machine. So how do you fix it?
Normally when someone encounters this issue it is because they have typed the name of the group they were looking for in the wrong place. When you compose an email it automatically puts your text cursor in the section to the right of the [To...] button like so:
If the group has been updated since when you last emailed it then using this section to add it will cause the old group to be emailed, not the new updated one. See below:
Clear the old group from the cache by clicking on the X to the right of the email:
The correct way to include a group is to go into the Address Book and search for the group from there. To get to the Address Book you can either click on Address Book from the menu up top:
Or you can click on the [To...] button like so:
From here you can add the group to the email by either double clicking on the group, or clicking on the group once, then clicking the highlighted [To ->] button as seen below:
Now you can send the email knowing that it will go to the right people from the updated email group.