Receiving HTTP responses to TCP pool (1 reply)

This one has me stumped - I'm trying to proxy SMTP connections however I'm
getting an HTTP response when I try to connect to port 25 (even though I've
done mode tcp).

This is the smallest subset that reproduced the problem - I can make this
work by doing "mode tcp" in the default section and then doing "mode http"
in all of the http frontends (not shown). But doing 'mode http' as default
and then 'mode tcp' in the smtp frontend definition seems to not work and
I'm not certain why.

global
daemon
maxconn 10240
log 127.0.0.1 local0
log 127.0.0.1 local1 notice
stats socket /var/run/haproxy.sock user root group root mode 600 level
admin
stats timeout 2m

defaults
log global
mode http
timeout client 30s
timeout server 30s
timeout connect 4s
option socket-stats

frontend mainfrontend
bind *:25
mode tcp
maxconn 10240
option smtpchk EHLO example.com
default_backend mxpool

backend mxpool
balance roundrobin
server mailparser-xxx 172.0.0.51:25 check port 25 weight 20 maxconn 10240
server mailparser-yyy 172.0.0.67:25 check port 25 weight 20 maxconn 10240