Plusnet email format

[#1]

I am slightly puzzled by the format of Plusnet's email addresses (eg: name@username.plus.com) as opposed to most other ISP's (eg: name@isp.com). I can't see any advantage in this format and it makes the username available which is surely a disadvantage!
Can anyone show me what I'm missing 

[#2]

Hi Dennis.
I've seen a similar format with other ISPs too - Freeserve certainly used to do it the same way.
It means that you can have multiple different mailboxes with the same username - you can have various different "names" before the username.

[#3]

Thanks James for your very quick reply 
I thought that may be the case, looks as though I may have to give the missus her own mailbox 
Dennis
ps. The developers seem to be very busy as there is no movement on my phone changeover yet.