Looks like with 3.11, enabling hotspot no longer adds the NAT rule for the SMTP server.
In 3.10, these rules are added (from /ip firewall nat print dynamic):
D chain=hs-unauth action=jump jump-target=hs-smtp dst-port=25 protocol=tcp
D chain=hs-auth action=jump jump-target=hs-smtp dst-port=25 protocol=tcp
D chain=hs-smtp action=dst-nat to-addresses=x.x.x.x to-ports=25 in-interface=bridge-hs
In 3.11:
I chain=hs-unauth action=jump jump-target=hs-smtp dst-port=25 protocol=tcp
I chain=hs-auth action=jump jump-target=hs-smtp dst-port=25 protocol=tcp
Keep up the good work with 3.x, mt.