On GNU/Linux Thunderbird ist my favourite GUI mail client, offering support for a variety of standards, including e.g. CalDAV (CardDAV in Beta), OpenPGP and S/MIME.
Not sure whether Chat should be part of it, currently offering Google Talk, IRC, Odnoklassniki, and XMPP (Twitter finally gone). Support for Usenet News is appreciated, though I enjoy Pan.
On GNU/Linux I thus consider GNOME Evolution as best alternative. (Haven't looked at the KDE suite for some time with Akonadi being too heavy last time I tried it on my simple machine.) Neither Claws, Geary, Sylpheed, or others offer such broad support, although those are very nice GUI clients.
I tried a lot of alternatives to thunderbird on linux for mail/calendar/contacts. Every single one had a severe issue at some point with something (timeline problems, losing events, can’t sync contacts, privacy issue for one paid app). Thunderbird is just wonderful though and deserves support
I want to like Kmail, but it's a bit too opinionated (e.g. having a setting letting you set to reply-to-all by default was removed because it is 'wrong', as well as having the button show up on messages without a roll-down) and just too crashy (akonadi...) and impossible to backup configuration (I spend a full day tracking all the various files/dirs down, no luck).
I think there's an A-rate tool hiding in Kmail and Kontact, but development is glacial and not very responsive feature requests. (I even asked for some help setting up a dev env to develop something myself but no replies.)
KMail used to be wonderful, but Akonadi has been a train wreck since day one. I had to switch to Thunderbird for work eventually because it just wasn't acceptable to not respond to urgent things because "sorry, my mail client decided to silently stop checking for mail again!".
> Neither Claws, Geary, Sylpheed, or others offer such broad support
But Geary works nicely on mobile phones, like the PinePhone. For me it serves a different niche than Thunderbird (which is my default desktop email-client).
Not sure whether Chat should be part of it, currently offering Google Talk, IRC, Odnoklassniki, and XMPP (Twitter finally gone). Support for Usenet News is appreciated, though I enjoy Pan.
On GNU/Linux I thus consider GNOME Evolution as best alternative. (Haven't looked at the KDE suite for some time with Akonadi being too heavy last time I tried it on my simple machine.) Neither Claws, Geary, Sylpheed, or others offer such broad support, although those are very nice GUI clients.
Further alternatives are listed in https://en.wikipedia.org/wiki/Text-based_email_client