QUICK FIX: Remove LegacyexchangeDN from the AD user properties and set the same attribute in exchange mailbox properties (see how here: https://www.petenetlive.com/KB/Article/0001468).
There are certain situations where the Global Address list display will show the legacyExchangeDN instead of the SMTP email address. The one condition where this is true is when you are viewing the Global Address List through Outlook 2003. The default behavior for Outlook 2003 is to use the EX address type for the Email Address column.
The default behavior for Outlook 2007-2010-2013-2016 is to use the SMTP email address.
The best thing you can do is look at the users attribute in the active directory if you are not sure, because the mail attribute should contain a SMTP proxy address.