IMAC: What is the SMTP outgoing mail server?

You need to get that information from your internet service provider if you are using an email address provided by them or, if you are only using iCloud, then the info can be found here:

http://support.apple.com/kb/HT4864

FWIW, in order to set up email, you need both an incoming and outgoing server address (in addition to your email address/user name and password). Those are the servers your email program uses when sending or receiving emails.