SIP: The Session Initiation Protocol is a protocol used for establishing sessions in an IP network. A session ranges from a simple two-way phone call right on up to multi-media conferences. When you make a phone call using NeoTalk (VoIP), SIP is used to establish the phone call.
SMTP: Simple Mail Protocol Transfer is an internet standard for sending emails across IP networks. First developed in 1982, there have been changes to the SMTP over the years and now we have ‘extended SMTP’, or ESMTP. The main thing you need to know is that if you’re sending an email you must have SMTP.