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WorkOS
The modern identity platform for B2B SaaS. The APIs are flexible and easy-to-use, supporting authentication, user identity, and complex enterprise features like SSO and SCIM provisioning.
I have tried several, and liked none of them. I'm currently on Geary, but it's lacking in functionality, and it has things like search results being a bit different upon each of my searches. Starred messages cannot be shown on top. Eyeroll.
I think Evolution and Thunderbird are the top contenders, and of the self-hosted ones, Roundcube.
https://wiki.gnome.org/Apps/Geary
https://roundcube.net/
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