Sorry to be 'that guy'. I like Fastmail but maybe they could fix some of the irritating bugs we keep asking for resolutions on before embarking on fun protocol invention missions. Two killers:

(1) When forwarding an email there is no link between the forwarded email and the original thread (TOTAL TIME WASTER), when reported they "don't consider it a bug [because of SMTP ID handling in existing daemons]". Missing the point here so hard guys!

(2) When creating a new filter it doesn't have a checkbox to include matching emails. This means you have to manually go replicate the search again, for consistency, to get things where they should be.

Frustrated at lack of response from reporting these as a commercial user. Inventing new protocols should not be higher priority than fundamental annoyances that are wasting customer time NOW.