What dynamic rules are you referring too specifically? The only ones that are generated for SMTP are in NAT, not in the filter section. If you could provide a more complete diagram of your network we would be able to help a lot more.
You can also narrow down the rule by specifying the in-interface to the interface of your hotspot.
Your last tip has done the job!!!!!!!.
/ip firewal filter
add chain=forward action=drop protocol=tcp in-interface=wlan2 dst-port=25 comment="Block port 25 for hotspot users"
The configuration is very straight forward. Created the config with the Quick setup (ap) and created some rules to protect my local network from the internet. Added a VirtualAp (wlan2) on which I've created a hotspot (with the setup wizard)
Added rules (filter and NAT) to protect my local network from hotspot users.
And with your help added a filter to block port 25 for hotspot users.
Thanks!!!!!