Outlook (and in fact all Office programs) handles its Add-ins through registry information.
There is a registry value which determines whether a specific Add-in should be loaded or not.
That registry information is located under the LoadBehaviour key:

Administrator accounts have this setting under:

  HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Outlook\Addins\EmailInfoAddin.Connect\LoadBehavior
or
  HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Office\Outlook\Addins\EmailInfoAddin.Connect\LoadBehavior

User accounts could also have individual settings under:

  HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\Outlook\Addins\EmailInfoAddin.Connect\LoadBehavior

In some rare instances, Outlook determines to disable a particular add-in, in which case it changes the LoadBehavior value from the normal 3, to a different value, usually 2. Once the LoadBehavior has changed from the default 3, the next time Outlook starts it will no longer load that add-in. In the unlikely scenario this happens to the E-mail Info add-in, there are a number of ways to re-enable it :

Run installer again.

1 - Stop Outlook.
2 - Run installer.

Running the installer again will not modify any settings except it will change the LoadBehaviour value back to the intial state.
This will only affect the system wide value stored in HKEY_LOCAL_MACHINE.
The installer will not modify settings under HKEY_CURRENT_USER.

3 - Start Outlook.

Run Re-enable

1 - Stop Outlook.
2 - Run Re-enable.

The installer has added in the menu, just for this unlikely situation, an entry that should enable to modify the registry entry back to the intial state.
This will only affect the system wide value stored in HKEY_LOCAL_MACHINE.
The installer will not modify settings under HKEY_CURRENT_USER.

3 - Start Outlook.

Manually modify the registry entry

1 - Stop Outlook.
2 - Run regedit.

At the command line or in Start Menu, run regedit.exe.
Find:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Outlook\Addins\EmailInfoAddin.Connect\LoadBehavior
or
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\Outlook\Addins\EmailInfoAddin.Connect\LoadBehavior
(if running as a user) and modify the value back to 3.

3 - Start Outlook.

Enable through COM Add-ins in Outlook

NOTE: These steps apply to Office 2010. Other versions of Office will have very similar steps.

1 - Start Outlook.
2 - Select File menu.
3 - Select Options entry.
4 - Select Add-Ins entry.
5 - Under Manage: select COM Add-Ins and press Go....
6 - Select E-mail Info Add-in checkbox and click OK.