I have many times tried finding a modern replacement for Thunderbird, but it always ends without success.
There’s Nylas Mail, Hiri etc, but there has always been some needed feature (for me) missing.
Then I heard about Monterail, which basically is a functional theme based on a Thunderbird mock up. And just recently an Arc (theme) integration for Thunderbird has been released.
The two together makes Thunderbird quite pleasant to look at.
Here’s how you setup Thunderbird to work with mail and calendar from Office365.
sudo apt install thunderbird
Start Thunderbird, and go to Tools, Add-ons, Get Add-ons.
Search for Lightning and install the add-on.
Goto Ericssons github repository for exchange calendar and download the latest stable version. Go back to Tools, Add-ons, Get Add-ons, click on the gear icon next to the search and choose Install Add-on From File.... Navigate to where you saved the xpi plugin file.
Go to Edit, Account Settings, Account Actions and Add Mail Account. Enter your settings.
IMAP server: outlook.office365.com:993
SMTP server: outlook.office365.com:587
Go to the Calendar (lightning) tab, right click in the left menu pane under Calendar, chose New Calendar... and select On The Network.
Format: Microsoft Exchange 2007/2010/2013
Name: <Name of the calendar>
E-Mail: <E-mail account created in previous step>
Exchange Type: Microsoft Office365
Click Check server and mailbox, it will ask you for your password.
Folder base: Calendar folder
Finish the wizard, and you’re all done!