SMTP, which is short for Simple Mail Transfer Protocol, is the software that enables you to send out messages from your email addresses. Without this, you will not be able to use email apps or webmail. Once you send an e-mail, your SMTP server creates a connection to a DNS server to learn where the e-mails for the recipient domain are managed. Subsequently, it connects to the remote mail server and trades details. If the recipient mailbox exists, your server transmits the e-mail to the receiving POP/IMAP server and the latter delivers that email to the particular mailbox where the recipient can get it. If you wish to be able to send e-mails, you should always make sure that the website hosting company has the service with their packages. Even when you work with an online form that visitors use to get hold of you, you'll still need an active SMTP server for your hosting account for the form to operate.