Exchange 2007 distribution list not receiving emails from external senders

This will be well known to any Exchange 2007 admin but given that all my Exchange 2007 servers are SBS 2008 machines, and that distribution groups created using the SBS wizards are automatically granted this permission, I had not come across this one before.

I usually use the SBS wizards for the tasks they are capable of achieving since this is how the product was designed to be used.  However recently one of out clients merged with another comapny and their SBS is now effectively running two companies, this renders the wizards useless for one company as they have two domains and the wizards are only designed to work with one.  So I had created some distribution groups in Exchange Management Console rather than the wizards but they would not accept email from outside of the organisation, senders receiving a 5.1.1.

A quick search revealed there is a command to be run against Exchange 2007 distribution groups in the shell before they will accept mail from external senders, by default RequireSenderAuthenticationEnabled is true:

Set-DistributionGroup "GROUPNAME" -RequireSenderAuthenticationEnabled $False

to re-enable:
Set-DistributionGroup "GROUPNAME" -RequireSenderAuthenticationEnabled

and, to check status: 
Get-DistributionGroup "GROUPNAME" | Select-Object Name, RequireSenderAuthenticationEnabled