Exchange system has a lot of Mail Contacts - (items listed under Recipient Configuration -> Mail Contact in EMC).
Don't ask me why but many of these "contacts" have the same email domain as regular mailbox users - IE normail mailbox user is email@example.com and there are contacts that are firstname.lastname@example.org, email@example.com - etc.
So we are getting a 4001 alert by Exchange Availability because it is trying to look up free/busy information on these contoso.com addresses but fails because they are "contacts" and there is no actual mailbox.
Is there a way to keep the availability service from attempting to look up free/busy info on these accounts?
The process that how to get the free/busy information is that: first exchange will receive a request, then exchange will detect the user information related to the users' suffix. If exchange detect the users' suffix is external address. Then exchange will stop to respond the request. So you should tell us more information about your contacts.
Please use the cmd:get-mailbox -identity issue user | fl to get more information about these issue users.
Second where do you find the error? In IE or In event log? Please tell me.
You need to delete the duplicate addresses from the contacts, or the entire contact. An e-mail address should only be used on one object in the entire forest.
Contact can be used to allow F\B look up in another Exchange org and I'm pretty sure there is no way to prevent Exchange from referring to contacts due to this support.
I have a VBScript here that will find all duplicate addresses within a forest:
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Microsoft Infrastructure Architect, MCSE: M, MCTIP, Microsoft Exchange MVP