Couldn’t update the primary SMTP address because this mailbox is configured to use an email address policy. To disable the email address policy for this mailbox, run the command with the EmailAddressPolicyEnabled parameter set to $false.
This is command for disable someone using EmailAddressPolicy so you can modify the email address
Set-Mailbox domain\kellyn -PrimarySmtpAddress firstname.lastname@example.org -EmailAddressPolicyEnabled $false
Another way is to follow up the Policy to modify the e-mail address
mail flow > email address policies > the Policy e-mail address is using an alias, so just change the alias for change the e-mail address