Two domains on Exchange 2010

I've got a client with both an Accounting Firm and a Payroll Processing firm. Both domains are accepted by his Exchange 2010 server and their desktop users are able to send mail as either domain. I now need to setup some remote clients, mostly iPhones/iPads to be able to send and receive emails. Connecting isn't an issue and they can get their email, calendars, contacts etc. But how do I allow them to send out as the added domain? The server is on 1stcompany.com and 2nd company.com is an accepted domain. The phones and tablets are connecting remotely to 1st company.com via the Internet and want to send email as 2ndcompany.com. I guess I could set up an open relay on the server but don't want to do that for obvious security reasons. Solutions?