Add a new email address to mailboxes in bulk

This one liner reads a list of user names from a text file, fetches the current set of proxy email addresses, and adds a new “test.com” SMTP email address to each mailbox.

Get-Content Users.txt | Get-Mailbox |% {$_.EmailAddresses.add("smtp:$($_.SamAccountname)@test.com"); Set-Mailbox -Identity:$_.Identity -EmailAddresses:$_.EmailAddresses}