Copying comment from #7:
I have contacted fastmail support about their JMAP notification endpoint. Here's part of the help ticket that describes the problem:
According to rfc8620 the Session object returned by a
JMAP server must contain an eventSourceUrl field that
contains the url...:
" The URL to connect to for push events, as described in
Section 7.3, in URI Template (level 1) format [RFC6570].
MUST contain variables called "types", "closeafter", and
The use of these variables is described in Section 7.3."
[..] Right now I'm connecting to jmap.fastmail.com and the
eventSourceUrl is "https://jmap.fastmail.com/event/" [..]