If the your email address book is out of sync and won't correct itself, you can update the list manually so that everyone's email address is showing the most version and nobody will be missing from the list either.
Step-by-step guide on updating your email address list:
In Outlook, click the Send/Receive tab and then click Send/Receive Groups → Download Address Book...
Uncheck "Download changes since last Send/Receive" and select the Full Details radio-button if that option is available.
Click OK to start the download.
Once the address book has been refreshed, close and restart Outlook. It should now match the online version of the Global Address List.