Making email pings work (Healthchecks behind Swag)

I’m running healthchecks(.io) behind a swag reverse proxy using the provided subdomain proxy conf. But I can’t get email pings for healthchecks to work. I have setup an MX record for the subdomain. What else do I need to configure to have emails arrive at the healthchecks smtpd?

(In case that’s relevant: I also use an Anonaddy/ container, which is receiving emails successfully)