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6 months agoSnappyMail seem to be a fork of Rainloop and both Rainloop and Snappymail appear to allow multiple providers - https://snappymail.eu/
Cypht seems to be a similar solution where you selfhost a webserver that acts as a web client to external email providers - https://www.cypht.org/documentation/
I find nothing about push notifications for either of those solutions though, and I’m not sure about how much the webclients cache.
Their site works fine without allowing javascript, that way it turns into quite a simple thing too!