The smallest element of Active Directory is the Domain. Each domain is controlled by a domain controller. This domain controller stores user account information, permissions and some basic contact information for that domain. A domain is always part of only one greater forest. A forest is comprised of one or more domains. Here the domains automatically setup trust relationships so they can assign permissions between the domains.

Domains (individual DCs) do not store information from other domains. However, each domain maintains a least one complete database of information from all the forest's domains. This is called the Global Catalog Server (GC).

Both a domain controller and a global catalog server both are separate LDAP servers:
Domain Controller :389  (ssl 636)
GC :3268 (ssl 3269)

If you would like to read only one domain's worth of information you can connect directly to that domain controller (DC). If you need to read from many domains (whole forest) you'll need to read from a Global Catalog Server (DC that contains the GC).

The GC ports are read only. Only the Domain Controller ports can be written to (389/636).

Active Directory with Exchange 200x

Exchange 200x is a MAIL SERVER. It uses Active Directory to store it's configuration data, account information etc. When Ex200x is loaded on AD, it modifes the LDAP schema of Active Directory to add Exchange specific attributes.  When syncing Exchange 200x, what you are really doing is reading/writing to the underlying AD DCs.  When writing to an AD destination (and this includes one with Ex200x loaded), you must still specify the IP address of a Domain Controller, not the Exchange Server itself.

When creating a new SimpleSync connection, the Destination Map Template you select determines if the AD objects created will include Ex200x attributes (aka be mail enabled). If you want your SimpleSync connection to create AD Exchange mail enabled objects, choose a destination map template that includes 'mail enabled' in the template name.