Exchange 2013 Change Primary SMTP Email Address
You might get the following, "Couldn't update the primary SMTP address because this mailbox is configured to use an e-mail address policy."
Here's how to fix:Set-Mailbox foo.user -PrimarySmtpAddress
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-EmailAddressPolicyEnabled $false
This will ADD the email address as the Primary SMTP Email Address and keep the current email address as a receiving email address.
Or if you need to set all the addresses for one mailbox all at once (the captial SMTP is the primary smtp address and the lowercase smtp is the additional smtp email addresses):
Set-Mailbox foo.user -EmailAddresses smtp:foo.user@domain1, smtp:foo.user@domain2, SMTP:foo.user@domain3 -EmailAddressPolicyEnabled $false