How a single exchange on a domain can be use by other domains

Subject: How a single exchange on a domain can be use by other domains ?

Dear all,

first of all thanks a lot for visiting for this question.

firstly I present current layout.

1) I have a domain .  e.g.,      

      domain name: domainA.B.C  ,  

     Exchange Server 2013 FQDN:     EXS.domainA.B.C  (192.168.1.5)

     domain controller is   :     DC.domainA.B.C     (192.168.1.1)

     IP range:     192.168.1.0/24

    users examples:       Umer@domainA.B.C,  Ali@domainA.B.C  

2) I have other 10+ independent sites  with different domain and different IP range. 

all sites connected though VPN. and can ping each other.

 

sites configuration example.

2.1) site1 

          domain name :     domainE.F

          Domain controller name:     DC.domainE.F  (192.168.2.1)

         IP range :   192.168.2.0/24

         user example:    Ahmed@domainE.F ,    Fahad@domainE.F , ....




now situation is that

my Main Office has only exchange server 2013 installed and all users and department on main office have their emails.

Q1: I want other sites use this exchange server (MXS.domainA.B.C) .

Q2: what are possible methods can be ?

Q3:  can I use single Exchange server (MXS.domainA.B.C) to store mailboxes of other sites and other sites use this Exchange server with their user names.

    Ahmed@domainE.F ,    Fahad@domainE.F   but in  MXS

what I tried :

I added "accepted domain" as "domainF.E"

I added emails policy and add email address format  "smtp@domainF.E" and applied

but I could not find way to add mailbox Ahmed@domainE.F ,    Fahad@domainE.F 

and what else I need to do and why i don't find domain: "domainE.F" in "MXS.domainA.B.C" mailbox ?

Thanks a lot in advance for helping me.



note: 

1) all networks (192.168.1.0/24, 192.168.2.0/24, ..... ) can ping each others.
2) all domains are separate. does not belong to single forest.  in other words there were separete sites and now connected thorugh VPN and can ping back and forth.