The following instructions step through the process of configuring your Office 365 Exchange Online account with Apple Mail for Mac OS X 10.11 (El Capitan).
*Note: Mac OS X 10.11 (El Capitan) is still in the the Early Adoption phase of the OIT Software Lifecycle. As such, there may be minor issues when using the Apple Mail program.
Open Apple Mail.
*Note: If this is your first time opening Mail, go to step 3.
From the Mail drop-down menu, select Add Account...
Select Exchange, then click Continue.
Enter a display name for your account in the Name field.
Select the apps you wish to sync, then click Done.
Next, you'll want to add your email alias (e.g. firstname.lastname@example.org) to your Account in Apple mail. This will prevent you from receiving your own message when you click reply all in an email message.
Select the Mail drop down menu, then choose Preferences.
In order to edit Aliases connected to your account, select the Accounts icon from the toolbar and be sure to select the Exchange account if you have more than one account listed. In the Outgoing Mail Server drop-down menu, select None.
Click the General icon in the toolbar to open a dialog box that will save changes.
Alternatively, you can select the drop down in the Alias field, select Edit Aliases, then use the + and - icons to add or edit email aliases. Click OK when finished to save changes.
In the Outgoing Mail Server drop-down, select Exchange.
Again, click the General icon in the toolbar to save changes. Close the preferences window.