Anything close to a Missive-like email client?

Hey all, Just trying to cut costs anywhere I can - my team and I use Missive and were wondering if there is anything similar or better that's open source? I haven't seen much yet. I'm playing with (and contributing to) extending https://www.agent-native.com/ 's mail app - which supports only Google sending, and that's all I need, so I don't need to manage DKIM, servers, etc. (The UI kinda sucks, but I like his overarching mission of bringing all these different tools into an extendable app - like the 'Clips' app he's working on, as I need to replace my Zight plan next year after our contract is up.) The UI of Cloudflare's Agentic Mailbox is neat, but they launched it once and haven't updated it since (https://github.com/cloudflare/agentic-inbox) - I may take some AI workflows from them, but yeah. Just seeing if anything is out there for Google-native open-source mailbox apps? Thanks in advance!