We experience problems with responding to emails via the warmshowers mail gateway, mean answers to existing conversations (messages+mid.XXX@reply.warmshowers.org) send via an ordinary mail client (gmail Android, Thunderbird) were not transfered to warmshowers internal message system.
We could reproduce the problem with different hosts/messages and always had to double check, if the message was delivered by logging into warmshowers. For a workaround we just use the warmshower internal message system.
Problem persist for us since 05.04.19 ~16:00 PDT
Happy to provide more details.