Hello.

This Works perfectly fine:
Enable-Mailbox -Identity 'domain.com/Users/Book, Mac' -Alias 'mac.book'

But this does not:
$user = "Book, Mac"
$identity = 'domain.com/Users/$user'
$alias = "mac.book"

Enable-Mailbox -Identity $identity -Alias $alias

Is it not possible to use variables with Enable-Mailbox?

Yes It is possible.

Try this...

$user = "Book, Mac"
$identity = "Domain.com/Users/" + $user
$alias = "Mac.Book"

Thanks.

Thank you for responding. I'll give this a go