Hi Louis,
Ask your ISP. As I mentioned earlier, most ISPs have a different (or "real") name for the mailbox for domains they host.
Eg.
Let's say we have an ISP here called IHUG (this is a real ISP here in OZ).
Then say they host a domain called sellersmarket.com.au
Let's say the account with IHUG is called smarket and they were provided with a mailbox called smarket@ihug.com.au
If they created an allusers mailbox for sellersmarket.com.au, the mail server at IHUG must accepts mails in the format anyuser@sellersmarket.com.au and drop them into a mailbox for you to collect. Usually, most ISPs would create an alias, using your example, allusers@sellersmarket.com.au which actually refers to smarket@ihug.com.au.
So, for your SME server to collect the emails from the ISP, you will actually need to collect from the mailbox called smarket@ihug.com.au and not from the mailbox called allusers@sellersmarket.com.au (which your SME server will treat as local).
However, your ISP might do it differently. Check with them. Also, while you are in conversation with them, see if they will be willing to add an additional mail header line to the e-mails for your domain (eg. X-Delivered-To). This will be useful when sorting out your mails that may have had their headers modified / removed or are part of mailing lists, etc. due to the limitations of multidrop mail systems.
Regards,
Kelvin