Reserving a room in Outlook with a different email address

We have 3 rooms we reserver with Outlook: Line1 Line2 and Line3

This works fine when using our Office 365 domain account (user@domain1.com).  But, we have a few outside users that use another domain with a Pop email service (domain2).  So when they make a meeting and invite Line1, for example, using their user@ domain2.com address the meeting invite goes out OK but the meeting never shows up on Line1's calendar.  This results in a double booking.

They need to use the domain2 address in order to not confuse attendees.  Is there a workaround for this?

Jim