Regedit, find:
Outlook 2007
[HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Security]
Outlook 2010
[HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Security]
Outlook 2013
[HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Security]
Outlook 2016, Outlook 2019 and Office 365
[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Security]
Level1Remove string
Value .cer;.exe;.xxx for the stuff you want to unblock.