Make sure the mailbox with bit 304 has a COS that uses bit 304.
Then, on the left column, go to Unified Messaging > UM User Licensing and click "Update UM User Licensing"
After that you won't see the error about the mailbox not being licensed - assuming you have enough licenses.
All three users should get the email, I have never seen it be an issue where one user gets it but others don't. I have seen it go to spam quite often, though.