Eu suspeito que este é um Add-In. Veja como desabilitar suplementos no Outlook 2013:
To access the Outlook add-ins management window, open Outlook and go
to File -> Options (for Outlook 2013) and go the Add-ins section.
You can’t remove Outlook add-ins directly here, but you can remove
them by clicking on the “Go” button located after the “Manage COM
Add-ins” text. This will popup a new window which will show all
add-ins (enabled and disabled).
Here you can locate the desired add-in and remove it from Outlook. You
can remove the Outlook add-in either by un-checking its check-box or
by selecting it and clicking on the Remove button.
Attention: if you remove an Outlook add-in using the Remove button,
the add-in will be removed completely from the list of available
Outlook add-ins. If you later decide to enable it back, it may be
tricky to find its library file.
It is important to know that, if you remove Outlook add-ins using this
method, the add-ins are not actually deleted from your computer: they
are only removed from the list of available Outlook add-ins. They will
not load in Outlook anymore, but their add-in files are not erased
from your computer.
Fonte: link