Thunderbird vs. Outlook
What I write here is some thoughts that I have after using Thunderbird and Outlook:
It is a free software. However, I am amazed on how improvements have been constantly made throughout revisions over time. The most current version is 45.4.0. I will pick up a few points upon using Thunderbird and share with you guys.
Configuring mail account
It is still not picking up server host name upon mail account configuration if your host name is not in Mozilla’s server name database or if you do not use a common server name such as mail.domain.com. Rather, you need to click on Manual Config button to further modify. However, SSL/TLS port settings can be auto-discovered.
Built-in Lightning feature
The Lightning Calendar plug-in is now integrated. We do not have to install this plug-in manually. Configuration steps are still the same that you need to find out the actual CalDAV URL of your CalDAV service to make it working. However, contact plug-in is still not included, such as SOGo Connector. Maybe one day it will.
There are 56 language pack add-ins for users speaking different languages to install. File size of each language pack is small. Make sure you go to Config Editor to switch language. Simply installing the extension will not work. However, there is one downfall that language pack is not working throughout different versions of Thunderbird. You need to find language pack that matches your Thunderbird version.
Outlook 2016 has been released for months. I think Outlook has a better UI layout than Thunderbird. As you know, Outlook.com now has Exchange server as its back-end server. Therefore, you can sign in your Outlook.com account easily under Outlook. This is a good news.
Account configuration other than Exchange-based account
I tried configuring my own on-premises mail server on Outlook and the auto-discovery is working perfectly. All I need to enter is my e-mail account and password. This is really amazing.
Lack of support to CalDAV and CardDAV
Again, Microsoft still ignore integrating CalDAV and CardDAV client functionalities into Outlook. This is a bit disappointing. Luckily, Microsoft opens a backdoor for add-in developers and there are add-ins for contact, calendar and task. These sync tools are gSyncit, EVO Collaborator for Outlook, DAV Collaborator and etc.
Language pack in Office365
Just some time after Microsoft released Office365, subscribers needed to install the whole Office suite in different language on Office365 portal website in order to use change language. After a few months, I found something changed that we can solely install language pack instead. That is a plus. For non-Office365 users, the only choices they have is to purchase language pack or go through open license program.
The above are some thoughts I feel about both Outlook and Thunderbird. Free software is cool but support wise is not as complete as paid software. If you use e-mail software for work, then Outlook seems like a better choice. If you use it for personal stuff, then Thunderbird is not a bad choice because it improves itself gradually for which you have no control over it and just take your time for new features to be released.