Thanks Derek, 

Can I ask why we can't share across Domains?  

I think ill need to extract all customer data from the user whilst in Domain A  then move the user across to DomainB and extract all the supplier data. 

However we are getting still getting suppliers emailing the wrong domain as not updating either systems.  

If only there was a SMARTER solution to let me drag straight into DomainB. yeah I can forward but that's messy when want to reply to email. 

Please note the following article regarding forwarded emails:

SM treats forwarded emails differently than other mail server solutions.

Anne,

It doesn't have to be "messy" with replies. You can set up a content filter for the account that sends a message to the sender notifying them of the change and forwarding the email to the new mailbox. You can also use an autoresponder if you like, and set up automatic forwarding. You could even set up an event to do the same things. 

Regardless of how it's done, replying from that new email address isn't (or shouldn't be) a surprise. You may also want to consider a footer for the domain that notifies people of whatever organizational change is being made, sort of like companies do when they're going through a re-branding. But that may be more than you want to do at this time.

As for "why" -- domains are essentially organizations. Sharing between orgs requires permissions and access controls, which can be complex to manage. Systems like Exchange have that built in through AD and ACLs. SmarterMail, and other mail servers, treat domains as their own entities or organizations. So, for the time being, sharing cross domain isn't possible, though it's something we're looking at implementing in the future.