SMTP, which is short for Simple Mail Transfer Protocol, is the software that permits you to send out emails from your email addresses. Without it, you won't be able to use e-mail apps or webmail. Whenever you send an e-mail, your SMTP server creates a connection to a DNS server to learn where the emails for the recipient domain name are handled. Then, it creates a connection to the remote mail server and exchanges details. If the recipient mailbox is accessible, your server sends the message to the receiving POP/IMAP server and the latter brings that message to the specific mailbox in which the receiver can see it. If you want to be able to send emails, you should always make sure that the hosting company offers the service with their packages. Even when you use an online form that site visitors use to make contact with you, you will still require an active SMTP server for your hosting account for the web form to work.