Some of the advantages of using custom attributes include:

• You avoid extending the Active Directory schema.
  
  
• The attributes are created by Exchange Setup.
  
  
• You can use the Exchange Administration Center (EAC) or the Exchange Management Shell to manage the attributes. You don't need to build custom controls or write scripts to populate and display these attributes.
  
  
• The attributes are filterable properties that can be used in the Filter parameter with recipient cmdlets such as Get-Mailbox. They can also be used in the EAC and the Shell to create filters for e-mail address policies, address lists, and dynamic distribution groups.
  
  

Multivalued custom attributes

In many Exchange deployments, creating an e-mail address policy for all recipients in an OU is a common scenario. The OU isn't a filterable property that can be used in the RecipientFilter parameter of an e-mail address policy or an address list.

Note:
Dynamic distribution groups have an additional parameter that you can use to restrict it to recipients in a particular OU or container.

If the recipients in that OU don't share any common properties that you can filter by, such as department or location, you can populate one of the custom attributes with a common value, as shown in this example.

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Get-Mailbox -OrganizationalUnit Sales | Set-Mailbox CustomAttribute1 "SalesOU"

Now you can create an e-mail address policy for all recipients that have the CustomAttribute1 property that equals SalesOU, as shown in this example.

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New-EmailAddressPolicy -Name "Sales" -RecipientFilter { CustomAttribute1 -eq "SalesOU"} -EnabledEmailAddressTemplates "SMTP:%s%2g@sales.contoso.com"

When creating dynamic distribution groups, email address policies, or address lists, you don't need to use the RecipeintFilter parameter to specify custom attributes. You can use the ConditionalCustomAttribute1 to ConditionalCustomAttribute15 parameters instead.

This example creates a dynamic distribution group based on the recipients whose CustomAttribute1 is set to SalesOU.

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New-DynamicDistributionGroup -Name "Sales Users and Contacts" -IncludedRecipients "MailboxUsers,MailContacts" -ConditionalCustomAttribute1 "SalesOU"

Note:
You must use the IncludedRecipients parameter if you use a Conditional parameter. In addition, you can't use Conditional parameters if you use the RecipientFilter parameter. If you want to include additional filters to create your dynamic distribution group, email address policies, or address lists, you should use the RecipientFilter parameter.

In this example, the mailbox for Kweku will have ExtensionCustomAttribute1 updated to reflect that he’s enrolled in the following educational classes: MATH307, ECON202, and ENGL300.

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Set-Mailbox -Identity Kweku -ExtensionCustomAttribute1 MATH307,ECON202,ENGL300

Next, a dynamic distribution group for all students enrolled MATH307 is created by using the RecipientFilter parameter where ExtensionCustomAttribute1 is equal to MATH307. When using the ExtentionCustomAttributes parameters, you can use the -eq operator instead of the -like operator.

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New-DynamicDistributionGroup -Name Students_MATH307 -RecipientFilter {ExtensionCustomAttribute1 -eq "MATH307"}

In this example, Kweku’s ExtensionCustomAttribute1 values are updated to reflect that he’s added the class ENGL210 and removed the class ECON202.

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Set-Mailbox -Identity Kweku -ExtensionCustomAttribute1 @{Add="ENGL210"; Remove="ECON202"}