The Simple Mail Transfer Protocol (SMTP) is an email sending protocol. Due to abuses by spammers, SMTP servers are now almost never open relays. SMTP uses the URI "mailto:", which is supported by most Browsers. Below are addresses of SMTP servers on which registered access is freely given (each host has their own method of registration).
[IANA Port assignment = 25]
Note that most ISPs allow unregistered access to their own SMTP servers when connecting from one of their own IP addresses.