Problem: Although the installation of MailStore's Outlook Add-in was successful, it is not displayed in Outlook.

Solution: This can happen due to various reasons:

• The MailStore Outlook add-in might have been disabled by Outlook, e.g. due to loading times.
  In Outlook click File > Manage COM Add-ins (Slow and Disabled COM Add-ins) and check whether the
  MailStore Outlook Add-In is listed. When it is listed, enable it permanently.
  The Add-in can be enabled permanently by using Group Policies, in this case please read this MailStore Article.
  
  
• Check whether the MailStore Outlook Add-In is listed in Outlook Options > Add-Ins. In case the Add-In is not listed,
  click Manage > COM Add-Ins > Go and verify that the Add-In entry is checked. Is the Add-In not listed in
  COM Add-Ins navigate to Outlook Options > Add-Ins > Manage > Disabled Items > Go and ensure
  the Add-In is Enabled. Afterwards, verify that the Add-In entry is checked in COM Add-Ins.
  If the Add-In is not listed in COM Add-Ins and in Disabled Items open
  Outlook as an administrator, navigate to Outlook Options > Add-Ins > Manage > COM Add-Ins > Go > Add...
  navigate to the installation directory of the Add-In and add the file MailStoreOutlookAddin3_x64.dll or
  MailStoreOutlookAddin3_x86.dll, depending on the bittness of your Outlook installation.
  You may have to re-install the Add-In so the DLL files will be registered.
  
  
• The load behavior of the Outlook Add-In is controlled by registry keys.
  When using a 64-bit Office or a 32-bit Office on a 32-bit Windows
  the value of the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Outlook\Addins\MailStoreOutlookAddin3.AddinModule\LoadBehavior has to be 3.
  
  When using a 32-bit Office on a 64-bit Windows the key can be found in the path HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Office\Outlook\Addins\MailStoreOutlookAddin3.AddinModule\LoadBehavior
  
  In case these keys exist in
  HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\Outlook\Addins\MailStoreOutlookAddin3.AddinModule\LoadBehavior ,  the keys in HKEY_LOCAL_MACHINE  still will be preferred and have to be adjusted there.
  
  
• Check whether the MailStore Outlook Add-In is listed in Outlook Options > Add-Ins > Manage >COM Add-Ins > Go and Load Behavior is either "Not loaded. Certificate of signed and load at startup COM Add-in is not in trusted source list." or "Not loaded. The user selected to disable macros.". In this case the user has  set not set the setting Outlook Options > Trust Center > Trust Center Settings... > Macro Settings and has set Apply macro security settings to installed add-ins and has disabled the Add-In during the start of Outlook. To load the Add-In lower the macro security and enable the Add-In during the start of Outlook or disable the setting Apply macro security settings to installed add-ins.
  
  
• By default the Add-In integrates into the Outlook Ribbon as main tab. Navigate to Outlook Options > Customize Ribbon > and verify that there is a MailStore entry in Main Tabs and that this entry is enabled. In case, there isn't, you can add the Add-In with Choose commands from > Main Tabs > MailStore > Add or Customizations > Reset. You are free to adjust the Add-In according to your needs here.
  
  
• Another reason is a corrupt Outlook profile. Try to create a new profile and see if the MailStore Outlook Add-in is displayed there.
  
  
• Check whether the .NET framework 4.8 is installed or enabled as a feature. For example, this is not the case with Windows 7 by default.
  
  
• Ensure you do not have disabled the MailStore Group Policy "Enable MailStore Outlook Add-In Toolbar by default". MailStore Group Policies are stored in the registry in
  HKEY_CURRENT_USER\Software\Policies\deepinvent\MailStore Outlook Add-in 2. These settings should not be modified manually, but only by using the MailStore ADMX templates.
  
  
• Deactivation by Microsoft Outlook Trust Center
  When Outlook opens, the application asks permission to load each add-in, automatically disabling any add-in that isn't signed.
  
  Following article maybe helpful:
  Microsoft article

Article-ID: KB20121128-1-EN (Deutsche Version)