You have a large no. of employees so it is hard to remember all email addresses.

Google Apps provide a feature to create Directory with the details of users in the domain.

You have just have to Enable Contact Sharing option from your Admin Console and you are ready with a directory of all employees with their Email addresses.

Go through the following steps to Enable Contact Sharing.

Step 1: Log in to your Admin Console and goto Google Apps.

Step 2: Select Contacts and goto sharing settings.

Step 3: There you can declare settings as needed.