Switchyard sounds like exactly what you are looking for, service that forwards email to XMPP. There's also milter-xmpp, which is a plugin to an existing mail server that supports postfix style mail-filters.
post
replies:
The Libervia email gateway might work:
https://www.goffi.org/forge/goffi/libervia-backend/src/branch/main/doc/howto_email_gateway.rst
https://www.goffi.org/forge/goffi/libervia-backend/src/branch/main/doc/components.rst
The how-to is a bit outdated (still work this way though), it can now start a component directly from the settings, without having to deal with CLI frontend, it's explained in components.rst
all 5 comments