A few days ago, the operations department at one of our customers raised a concern about the support in Exchange Server 2003 for resolving mail recipients against the set of AD contacts when some of them share the same SMTP address. "This is not allowed in AD, and you should fix your code to not add the same SMTP address multiple times to AD" he stated. What ? As "proof" he sent me a screenshot of an error he got when trying to add a new contact with a duplicate SMTP address using the AD Users and Computers admin tool:
This e-mail address already exists in this organization (c10312e7)
This is a known limitation in the Active Directory Users and Computers snap-in. To resolve this, do not create the contact with AD Users and Computers, but use the ADSI-edit MMC snap-in or the ADSI component from your code to add multiple AD contacts that must share the same SMTP address. The AD attributes to set are "mail", "mailNickname", "proxyAddress", and "targetAddress". Those fields can be modified to enter a SMTP address that exist for other contacts without error (more info at MSDN).
Note that you should not create Exchange mailboxes for any external AD contact, and never create AD contacts duplicating the mail addresses of internal users. This will lead to NDR errors during Exchange resolving and mail delivery. Also ensure that you use correct settings for the Exchange Recipient Update Service (RUS) on your AD contacts, as RUS will automatically create Exchange entries for your AD contacts when not disabled as applicable. Read more in this post.A related problem to Exchange NDRs for unreliable AD contact data, is the Outlook 2003 recipient cache. This autocomplete mechanism is heavensent when adding recipients that you send e-mail to often, but it is a p.i.t.a when your favorite recipients change their e-mail address or their data change in AD (the source of Outlook contact groups, a.k.a Exchange Global Address Lists). E.g. removing contacts from AD might lead to errors when Exchange tries to resolve cached recipients.
It is not easy to change the cached recipients list in Outlook using Outlook. The only option you have is to delete cached entries, but not many users have been able to find where to do that or clear the cache. It is actually quite simple, but not very intuitive; just select the entry you want to remove in the autocomplete suggestion list and press 'Delete' on your keyboard. The whole list of cached recipients is stored in a .NK2 file in your application data folder. Delete this file to clear the cache completely.
Read this article at Outlook Exchange for more info about the Outlook recipient autocomplete cache (Outlook .NK2 file). Also check out the Ingressor NK2 Management tool for editing and managing the .NK2 file.