A popular question on many forums goes something like this

Outlook isn't recognized by [application_name]. The error is "Either there is no default mail client or the current mail client cannot fulfill the messaging request. Please run Microsoft Outlook and set it as the default client." When I check the Default Programs, Microsoft Outlook is the default for everything.

First and foremost, when you receive this message while using Outlook 64-bit, try the 32-bit version of Office. While some applications and addins work with the 64-bit version, not all do. Unless you need to open large (2 GB) Excel and Access files, you won't benefit the 64-bit version. Wanting to use the 64-bit version because you use a 64-bit operating system isn't enough reason to use it, if you need compatibility between different applications, you'll need to stick with the 32-bit build.

Repeat: Avoid the 64-bit version unless you have confirmed the add-ins and utilities you want to use will work with it. There is no benefit to using the 64-bit version for typical users - Office is not going to run faster or better. Only those who work with huge (2GB) record sets and spreadsheets need the 64-bit version. The 32-bit version works fine on 64-bit Windows (do not use compatibility mode).

While many applications will work with the 32-bit version of Outlook, some programs will need to be updated before they will work with Outlook 32-bit. This is normal with all new versions of Office.

Note: If you are using an Office 2013 or newer Click to Run suite, add-ins should work with it. some add-ins may need to be updated, but Office or newer applications can run addins. This is the exact opposite of Office 2010 Click to run!

If you sync with a smartphone or pda, you need to confirm your device will sync with the 64-bit version of Outlook.

From Microsoft:

Office provides support for the 32-bit version of Office programs running on 64-bit operating systems by using WOW64, a compatibility environment provided by the operating system that allows a 32-bit application to run on a Windows 64-bit operating system. Using the 32-bit version of Office allows people to continue to use existing third-party add-ins for Office that are 32-bit. The 32-bit version of Office is the recommended option for most users, because it prevents potential compatibility issues with other 32-bit applications, specifically third-party add-ins that are available only for 32-bit operating systems.

Pros & Cons: 32-bit

• Many existing add-ins and programs work with it.
• Can co-exist with other 32-bit Office applications.
• Office has the same memory requirements as Office 2007 but even with 32-bit Office, you can open, edit, and switch among applications much faster on 64-bit Windows than on 32-bit Windows.
• Syncs using WDMC

Pros & Cons: 64-bit

• Supports larger file sizes - over 2GB in Excel and Access
• Most add-ins and Active X controls do not work with it
• Cannot co-exist with 32-bit Office family applications
• Syncing with Windows Mobile devices using WMDC NOT supported

"Click to Run" version

While the click to run Office suite is 32 bit, most add-ins will not work with it because it runs in a virtual space. Users need to install a regular 32-bit suite instead. I don't recommend the Click to Run version, except for users who need two versions of Outlook.