Also opening a mail that's 30 MiB in size (pictures) and has to be downloaded just freezes the entire UI.
I'd like to advise people to use other software, but I don't know of any perfect email clients:
* sylpheed/claws-mail has this locked UI problem
* mutt doesn't render images and has a steep learning curve
* thunderbird has software bloat, doesn't work on my low end hardware
And other recommendations?
Webmail is also mostly broken. I liked prayer, but it also doesn't render images IIRC and roundcube dropped the classic theme on new versions, and the new theme has less information density.
I like the classic HTML UI of GMail and OWA, though.
Since you mentioned mutt and it's steep learning curve, I recommend you another TUI MTA/mail app. It's nmail (https://github.com/d99kris/nmail). Simple, Pine/Alpine like interface, has great features (for example, saving mails in sqlite - can be viewed offline). Also the developer is active and friendly, in case you find bugs or have any proposals for enhancements.
https://www.claws-mail.org/index.php
It's lightning fast compared to Outlook, and very stable/reliable. Also available on Windows.
Outlook .pst files (both mail and contacts) also can be easily ported to Mbox or other standard formats, then imported into Claws Mail, by using the readpst utility which is part of libpst utilities, available on various Linux distros. On Debian it's part of pst-utils.
https://www.five-ten-sg.com/libpst/