We have two domains with AD two-way trust and Exchange GAL synchronization done via Forefront identity Manager 2010. We have a problem with meeting room reservation, user from domain A is trying to book meeting room from domain B (there is a contact object in domain A for this meeting room from domain B). The problem is that such room is not sending response confirmation email to user and meeting room is visible as tentative. I already set "externally secured" option on receive connectors in domain B:

exchange 2010 server - receive connector role: HubTransport, scoping exchange server in domainA IP port 25, security: tls and externally secured

3x exchange 2013 servers - receive connector role: FrontendTransport, scoping exchange server in domainA IP port 25, security: tls and externally secured

I am not sure whether this is enough configuration for receive connectors, what about receive connector  for hubtransport role on 2013 exchange servers? Should I also configure them, what ports?

Domain A: one exchange 2013 server

Domain B: one exchange 2010 server (meeting rooms hosted in db on this server) and 3 exchange 2013 servers in DAG, prepared to migrate mailbox db from 2010 server

Have you checked the settings and permissions on the meeting room to ensure that users in a different forest can book meetings?

Also, check the message tracking logs to see if room booking emails are failing delivery between the domains.

Are you using the same email domain for both forests?

Also, have you done tests with the mailbox moved to Exchange 2013?

Thanks.

get-calendarprocessing results below

RunspaceId                          : ed128c74-0fa6-4c1c-aaf1-0d0726a9bc85
AutomateProcessing                  : AutoAccept
AllowConflicts                      : False
BookingWindowInDays                 : 360
MaximumDurationInMinutes            : 1440
AllowRecurringMeetings              : True
EnforceSchedulingHorizon            : True
ScheduleOnlyDuringWorkHours         : False
ConflictPercentageAllowed           : 70
MaximumConflictInstances            : 50
ForwardRequestsToDelegates          : True
DeleteAttachments                   : True
DeleteComments                      : True
RemovePrivateProperty               : False
DeleteSubject                       : False
AddOrganizerToSubject               : True
DeleteNonCalendarItems              : True
TentativePendingApproval            : True
EnableResponseDetails               : True
OrganizerInfo                       : True
ResourceDelegates                   : {xxx}
RequestOutOfPolicy                  : {}
AllRequestOutOfPolicy               : False
BookInPolicy                        : {}
AllBookInPolicy                     : True
RequestInPolicy                     : {}
AllRequestInPolicy                  : False
AddAdditionalResponse               : False
AdditionalResponse                  :
RemoveOldMeetingMessages            : True
AddNewRequestsTentatively           : True
ProcessExternalMeetingMessages      : True
RemoveForwardedMeetingNotifications : False
MailboxOwnerId                      : xxx
Identity                            : xxx
IsValid                             : True
ObjectState                         : Changed

I used get-messagetracinglog cmdlet and it does not show anything for senders from external domain. 

We use two separate email domains for each forest.

I created test room mailbox on exchange 2013 db and it works - accepts meeting requests. Tried also get-messagetracinglog and it shows senders from external domain

I migrated one meeting room from exchange 2010 to exchange 2013 but now even internal users cannot book meetings, maybe it need some time to fully sync this migration. If this (migration meeting rooms to 2013) resolve this issue how I can track what is causing this issue on exchange 2010?

Take a look at this article: http://ehloworld.net/?p=277.

I know that receive connectors should be configured with externally secured options, the question is which exactly receive connectors should I configure and on which servers?

As I wrote yesterday: I migrated one meeting room from exchange 2010 to exchange 2013 and now external users can book meetings, confirmation email is also received. How I can track what is causing this issue on exchange 2010?

But another issue after migrating this meeting room from 2010 to 2013 exchange is that in external domain users cannot see free\busy information about this room - how to troubleshoot this? Free\busy information are displayed for all rooms hosted on exchange 2010

Check your EWS URLs on the server that hosts the room mailbox which users cannot see free/busy information for:

Get-WebServicesVirtualDirectory

Check your DNS and firewall configuration to ensure that these URLs are accessible from the clients.

Also check that your availability service is configured correctly: https://technet.microsoft.com/en-us/library/bb125182(v=exchg.150).aspx

Thanks.