Are Exchange (email) accounts "live" upon creation by admin, or only after first login by user?

I create new Exchange accounts (for email only) for certain external vendors.

I also set them up with access to a few different internal business systems, all using their new email address as their username.

Each of these internal systems sends an automated setup email to the vendor, with a temporary password.

So — my (admittedly rookie) question is this....

Is their new Exchange/Outlook email account "live" and able to receive emails as soon as I create their Exchange account? Or is it only "live" after they login for the first time and change their password?

I ask, because I'd like create all their internal systems accounts immediately after I create their Exchange account — but I don't want the automatic notification emails to bounce. I was hoping that those emails would be waiting in their Outlook inbox when they login for the first time...

Thanks for any help!