What is the difference between Owl and ExQuilla? Which one should I use?

Short answer:

• If you have used ExQuilla for the past few years, you may continue to use it.
• If you are a new user, try Owl first.

Long answer:

ExQuilla

ExQuilla exists since many years and has a reasonably large user base. It has received many bug fixes over time, and it's consequently a fairly mature product.

ExQuilla requires the server to support the EWS protocol. Some servers have it enabled, others do not. ExQuilla cannot use any other protocols.

ExQuilla is based on an old technology (called XPCOM), which Mozilla has deprecated years ago. Firefox extensions of this type were already killed by Mozilla in 2017. Thunderbird still supports them as far as possible, but they are subject to heavy breakage caused by Mozilla actively removing the fundamental technologies it's based on. This happens piece by piece, but more and more. We are never sure whether we'll be able to support the next major version of Thunderbird. The future of ExQuilla is uncertain, and it costs a lot of work to keep it functional.

Owl

Owl was created as a response. It was built from the start as a "MailExtension" (like WebExtensions, but for Thunderbird). This new type of addons is supposedly the future.

Owl was one of the first MailExtensions, and the first large one. Consequently, we had to build a lot of the infrastructure and APIs, because Thunderbird did not have them when we started. We built the APIs to show mails in Thunderbird, download attachments, to add contacts to the address book, to populate the Lightning calendar, and to send mail. None of these APIs existed when we started, so we had to build them.

As a young product, Owl has bugs. You could say Owl is in his teenager years and still learning the ropes. We test it as well as we can, but often users encounter bugs that we do not see, and these are very hard to fix. It often depends on the Exchange server configuration, the specific situation and circumstances and other factors. If you run into such a bug, the more information you can give us to reproduce it, the better.

Last but not least, Owl supports 2 different protocols to speak with Exchange servers: OWA and EWS. If your server does not have EWS enabled, but you can use Outlook Web Access, then Owl will probably still work. For such users, Owl is often the only way to get to their email in Thunderbird.

Owl also supports Multi-Factor Authentication (MFA).

Licensing

ExQuilla and Owl are separate products. They need to be developed and maintained separately. To reflect that work, they are licensed separately. They both cost the same, but you cannot transfer your license from one product to the other, but you need to buy a new license. However, both offer a 1 month trial period, so that you have enough time to see which product works better for you. If in doubt, or both are good for you, go for Owl.

Summary

• If you've already been using ExQuilla since a long time and are happy with it, you can use it, as long as it exists. You can try out Owl at a moment of your choice. You will probably see things that are a little different, but hang on and let it mature.
• If you come here wondering which one of the two to try, try Owl first.