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.
Switchyard seems to be geared towards internal forwarding of emails within a server.
But have you tried it? I might have a usecase for it other than what op wants and am wondering how usable it is.
I haven't used it, initially heard of it because I follow the developer's blog. Here's the blog post about it.
Cool to read that they plan to have it fall back on email when the address doesn't work via a xmpp.
I am curious to try it, but when thinking about it I realized my OIDC SSO solution uses email addresses as identifiers, which makes this a bit more complicated 😓
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The how-to is a bit outdated (still work this way though), it can now start a component directly from the settings, without having to deal with CLI frontend, it's explained in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
XMPP
XMPP (aka Jabber) is the community-owned standard for real-time federated messaging.
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