My weird diversified life with three different email applications...
Security matters!
Support for PGP/GPG and IMAP/SMTP with SSL support is necessary.
Email is by a nautical mile the most abused service on the Internet. I want to know who I am exchanging mail with, and I do not trust Internet service providers to handle security properly. I digitally signed all my emails, and all data must be encrypted when exchanging sensitive information.
Sadly, many of my contacts don't bother configuring PGP/GPG or S/MIME to provide a layer of authentication and integrity checking to their emails. As a result, their emails can be altered or impersonated by system administrators and bad actors that gain access to servers. Many potential damages could be mitigated if they configure their email clients correctly in five minutes.
S/MIME
Configure S/MIME on Outlook, Apple Mail, Thunderbird, or whatever email client you use...
Create self-signed certificates at the very least, if you don't want to deal with certification authorities... people who exchange mail with you only have to set their trust in the certificate once and will notice if it changes or fails to validate.
PGP/GPG
Configure PGP/GPG locally on your Mac, Windows, or Linux to create your certificate and import it to your mail client. Outlook and Apple Mail need plugins, but most other clients (like Thunderbird) have built-in support for PGP/GPG.
Microsoft Office365 integration (email, calendar, and contacts)
The company I work for uses Microsoft tools, so I need them... Outlook would obviously fit my needs, but I hate it. Like most Microsoft tools, it's bloated with features I don't care about, and the UX is designed by people with a sense of taste I don't share. So if you like it, good for you, but I will not go near Microsoft productivity tools if I have better options.
I do not use Windows for anything other than games, so I don't even have an email client installed...
Apple iCloud integration (email, calendar, and contacts)
I use Apple Macs and iPhones in my personal life. I like native Apps like AppleMail, Contacts, and Calendar that come with MacOS, iPad, and iPhone. While in my professional life, I need the Microsoft ecosystem of Apps and access to the company contacts directory and calendars, in my own time, I need access to my personal ones in iCloud.
On the Mac, I use Apple Mail with “GPG tools” extension.
On my iPhone and iPad, I use mostly Canary Mail simply because it's my best bet supporting PGP/GPG. Unfortunately, apple Mail supports only S/MIME (I keep it on my devices for that reason; I may need S/MIME from time to time).
I use Linux on my main laptop...
I code and do a ton of system administration with my Linux desktop. That is what I take with me whenever I go on vacation or on business trips.
I use Thunderbird with the OWL plugin. It works well with Office365, iCloud, and Gmail.
Thunderbird has spent a few years in limbo but is now again being actively developed, and it is simply the best email client for Linux.