hello
what is smtp server in hotspot setup ?

Whatever you specify as the SMTP server will generate a dstnat rule for TCP/25 forcing all SMTP traffic to that IP address.
This is a workaround for users whose SMTP servers allow open relay from the ISP's own IP ranges, but block off-net relay. Hotspot operators can offer their own smtp relay, and transparent redirection onto that relay eliminates a lot of calls into the customer service department complaining that they can't send email "on your stupid service."

This is probably much less necessary in today's world because most mail servers require SMTP-auth (so their email client doesn't need any changes when working off-net) or else the users don't even use mail clients but web-based email instead.