SMTP (simple mail transport protocol) is the way you deliver messages from your computer to your isp. The address you'll need here, you have to get from your ISP. sometimes it'll be the same as the address for your incoming server, sometimes it won't. If you are only ever going to receive emails and never send then you could theoretically enter a false address (for example, just use the same as the incoming server) and it will let you progress but will generate an error if you do try to send a message.