Which Windows PowerShell command should you run?

Technet docs on Set-Mailbox now does reflect the WindowsEmailAddress parameter’s behavior.

https://technet.microsoft.com/en-us/library/bb123981(v=exchg.160).aspx

And the answer has to be C, not D, as D won’t change the current setup. By not using smtp: (lowercase) or SMTP: (uppercase) prefixes, and putting the onmicrosoft address first, executing D continue to make the onmicrosoft e-mail address the primary (sent-from) one.

Technet docs on both the EmailAddresses parameter and parameter:

** EmailAddresses **

The EmailAddresses parameter specifies all the email addresses (proxy addresses) for the recipient, including the primary SMTP address. In on-premises Exchange organizations, the primary SMTP address and other proxy addresses are typically set by email address policies. However, you can use this parameter to configure other proxy addresses for the recipient. For more information, see Email address policies.

Valid syntax for this parameter is []:,[]:…. The optional value specifies the type of email address. Some examples of valid values include:
SMTP The primary SMTP address. You can use this value only once in a command.
smtp Other SMTP email addresses.
X400 X.400 addresses in on-premises Exchange.
X500 X.500 addresses in on-premises Exchange.

If you don’t include a value for an email address, the value smtp is assumed. Note that Exchange doesn’t validate the syntax of custom address types (including X.400 addresses). Therefore, you need to verify that any custom addresses are formatted correctly.

To specify the primary SMTP email address, you can use any of the following methods:
– Use the value SMTP on the address.
– The first email address when you don’t use any values, or when you use multiple values of smtp.
– If it’s available, use the PrimarySmtpAddress parameter instead. You can’t use the EmailAddresses parameter and the PrimarySmtpAddress parameter in the same command.

To replace all existing proxy email addresses with the values you specify, use the following syntax: “[]:”,”[]:”….

To add or remove specify proxy addresses without affecting other existing values, use the following syntax: @{Add=”[]:”,”[]:”…; Remove=”[]:”,”[]:”…}.

** WindowsEmailAddress **

The WindowsEmailAddress parameter specifies the Windows email address for this recipient. This is a common Active Directory attribute that’s present in all environments, including environments without Exchange. Using the WindowsEmailAddress parameter on a recipient has one of the following results:

– In on-premises environments where the recipient is subject to email address policies (the EmailAddressPolicyEnabled property is set to the value True for the recipient), the WindowsEmailAddress parameter has no effect on the WindowsEmailAddress property or the primary email address value.

– In cloud environments or in on-premises environments where the recipient isn’t subject to email address policies (the EmailAddressPolicyEnabled property is set to the value False for the recipient), the WindowsEmailAddress parameter updates the WindowsEmailAddress property and the primary email address to the same value.

The WindowsEmailAddress property is visible for the recipient in Active Directory Users and Computers in the E-mail attribute. The attribute common name is E-mail-Addresses, and the Ldap-Display-Name is mail. If you modify this attribute in Active Directory, the recipient’s primary email address is not updated to the same value