Custom Address book

How are the members of your sales team going to use this list?  I ask because the first requirement should already be created (the GAL).  

For the second requirements, as much as I HATE to say this, apart from a proper CRM solution, your best bet may be a Public Folder for the shared 3rd party contacts.  You could also create Mail Contacts in Exchange containing the 3rd party address as the ExternalEmailAddress for the Mail Contact, then create an Address List containing all the Mail Contacts.  You could even share a Contacts folder from either a pre-existing or shared mailbox.  You have several options here depending on your requirements.

Currently, external email address has been created as Contact in Exchange.

We need a address book that include Global + external email address (only be accessed by users in Sales OU). Can it be archived ?

Thx

Yea that's no problem.  Just create an address book policy.  Instructions in a nutshell are below:

1. Assign a unique value to all sales staff in one of the CustomAttributes in Exchange
2. Create a new Address List for sales with a recipient filter of {CustomAttribute1 -eq 'sales'} or whatever you end up using
3. Create a new OAB containing the Address List created in step 2
4. Create an Address Book Policy containing the OAB and Address Lists created above
5. Assign the ABP to your sales staff

http://msexchangeguru.com/2016/03/03/abp-exchange/
https://technet.microsoft.com/en-us/library/hh529948(v=exchg.150).aspx

Just FYI...that's a large part of how a multi-tenant Exchange instance is created.

Current we have 250 mailbox with around 20 users in Sales team. Only the 20 users need the Global address + the contact. The others  only need Global address book.

After digesting the link, does it mean I need to create a custom policy without contact for  those general users while keep the default for Sales users to view the GAL + contacts ?

Thx

No and that's my fault, I forgot about the GAL when I was listing the steps.  The article I included has a better breakdown of the process:

1. Set the custom attribute for the users within the different department.
2. Create two new GALs for both Departments.
3. Create new address lists for both Departments.
4. Create new offline address books (OABs) for both Departments.
5. Create two new address book policies referencing the above GALs, address lists and OABs.
6. Assign the address book policies to users in each Departments

Default GAL will contain all users + contacts (external mail address) and this will be assigned to Sales team.

Create custom attribute for remaining users, only with users without the contact. Is it what you mean ?

Thx

Glad I could help.  Take care :)