Regular mailbox-enabled users are automatically added to your Global Address List (GAL). Frequently, your organization may also choose to use the GAL to store partner, vendor, customer, or other external contact information. This article will show you how you can accomplish that.
Entities that are external to your organization can be added to your GAL via external Contacts. Follow these steps to add external Contacts:
Start with your Office 365 admin portal - https://portal.microsoftonline.com - then, navigate to Admin > Exchange to get to the Exchange administration center.
Once in the EAC, click on the Contacts link:
When the Contacts are opens, click on the [ + ] button and choose Mail Contact (Mail User can be used to secure access to a resource, and it's not the option you want if you simply are adding an external contact to your GAL):
Fill out the new mail contact form and click on Save:
Display name, Alias, and External email address fields are required.
Important note on the Alias field: i recommend that you follow this naming convention when creating contact aliases:
where [alias] is a concatenation of the first and last name of the contact, so for a contact named John Boyd, the alias will be contact-johnboyd.
Following this naming convention it will be easier for you later to export your contacts and distinguish them visually from other types of objects. Strictly speaking, this is not necessary - but can be helpful.
If you get an error message when saving, click cancel and refresh the page - chances are your contact was added anyway :).
Once your contact is saved, you can add more details by selecting it in the table and clicking the edit button (looks like a pencil):
The contact details form allows you to enter their phone number, address, company info, and MailTips.
All fields are self-explanatory, with the exception of MailTips (and that one's pretty easy, too). MailTips can be a really useful feature for communicating some bit of info about the contact when that contact is added to an email message. For example, if you know that the contact is generally unavailable on Tuesdays and Thursdays, you could, in MailTips, enter that information and anyone sending a message to that contact from your organization will be automatically warned not to expect a response on those days. MailTips will be shown both in Outlook Web App and in Outlook 201x.
Entering a MailTip example:
Outlook Web App MailTip:
If you need to add contacts in bulk, you can use PowerShell, or create a ticket for assistance.