As of version 1.6, chasquid supports being deployed behind a HAProxy instance.
This is EXPERIMENTAL for now, and can change in backwards-incompatible ways.
In the backend server line, set the send-proxy parameter to turn on the use of the PROXY protocol against chasquid.
You need to set this for each of the ports that are forwarded.
Only PROXY protocol v1 is supported for now.
Add the following line to /etc/chasquid/chasquid.conf:
haproxy_incoming: true
That turns HAProxy support on for all incoming SMTP connections.