Outlook Plugins

What are Outlook Plugins

Outlook plugins (also confusingly called COM add-ins by Microsoft) are the most mature method of adding features and providing third party integrations into Outlook.

Outlook plugins provide a feature rich and robust method of adding features. Some of the key differences between Outlook plugins and Office 365 add-ins are:

 Advantages of plugins compared to add-ins

Flexible User Interface

Outlook plugins have many options to enhance the user interface, including:

  • Add items to the ribbon bar anywhere you want
  • Add whole tabs to the ribbonbar
  • Show a new pane top, left,right, bottom of the Screen
  • Show popups and popup windows
  • Show message boxes

Respond to events

Outlook plugins can automatically show or  do things when something happens in Outlook – an event. These events include:

  • New email arrives
  • Email is sent
  • New message is opened for composing.
  • Read an email
  • Edit an appointment

Provide background processing

Outlook can run some processes in the background. This means while the user is checking email, the plugin can be synchronising contacts.

Work with all items in Outlook

Outlook plugins can access any item in Outlook including:

  • Email
  • Appointments and Meetings
  • Contacts
  • Tasks
  • Journals
  • Folders

Works with all version of Outlook Desktop (e.g. Outlook 2003 onwards if you wish)

Outlook plugins can be created to work with all versions of Outlook which run on Windows desktop. Typically they are only created for Outlook 2007/2010 and updards these days, but technically they an be created with one version to run in all Outlook desktop versions.

Plug-in Disadvantages


Outlook plugins are installed like any windows application using a windows installer. This can get complicated when taking into account permissions, admin installations, Citrix type hosted desktop environments and many other factors. Compared to installing Outlook add-ins, installation can be more complicated.

Cannot run in Outlook for Mac

Plugins do not run in Outlook for MAC – and never will. If you need to run in Outlook for MAC,you will need to create an Add-in.

Cannot run in Outlook Web Access / Outlook for Mobile

Plugins do not run in Outlook Web Access or Outlook for Mobile – and never will. If you need to run in Outlook Web Access, you will need to create an Add-in.

Plugin development is different

Developing a plugin for Microsoft Outlook is not like developing a standard application. Outlook has some very special considerations which can catch out the unwary developer.

For example Outlook is a single threaded application. This means that if you try to do two things at once, its easy to crash Outlook.

Then there are the multiple versions of Outlook, each with different ribbons, extra features, etc. And finally you have to manage the process of creating and installing the plugin – for multiple operating systems and environments.

Plugin Development Considerations

Speed and Responsiveness

Many Outlook users spend much of their working day using Outlook. Any plugin which causes Outlook to slow down (or worse), will not be popular and may be quickly un-installed.

A key issue for Microsoft Outlook Developers to consider is that it is a single threaded application. This means it is not possible for Outlook to do two things at the same time. If you try to create a separate worker thread – you run the risk of crashing Outlook, so any work which needs to be carried out for the add-in happens ‘in process’. This means if the work takes a long time – Outlook will appear to be frozen. This is particularly an issue when carrying out functions like calling a web service. Delays which may be tolerated in a web browser are not welcomed in what is seen as a ‘Desktop’ application.

The User Interface

The user interface is probably the most important element in any application. Outlook offers options to add buttons to toolbars, and to add custom panels and forms, but options and features vary between the various Outlook versions – Outlook 2007, 2010, 2013, 2016 (32/64bit). So a key first question is which versions you will support?

The requirement for most add-ins is to support at least Outlook 2010, but there are surprisingly large number of Outlook users who are still using 2007. If we just consider supporting the user interface for these three versions –  Outlook 2007 uses the Office Ribbon Bar for items in folders but Toolbars for folders, and Outlook 2010 upwards uses the Ribbon Bar everywhere. Toolbars and Ribbon Bars use completely different methods for creating and tracking. Just supporting toolbars and ribbons in multiple versions of Outlook can get interesting!

Experienced Outlook Developers

Davton have been working with Outlook for 10 years. We are learning new things about Outlook and Exchange every week. When it comes to developing plugins, we have learned what works and what might cause problems. So when we build you a plugin – you don’t just get the features you requested –  you get our whole stack of experience making sure we design a robust stable solution.

Phone David

Call me or email me for an informal discussion on how we can help you build you application.
+44 1482 869891 (UK office hours)
or email david@davton.com