SMTP, which stands for Simple Mail Transfer Protocol, is the software that enables you to send messages from your email addresses. Without this, you will not be able to use email apps or webmail. When you send an e-mail, your SMTP server creates a connection to a DNS server to find out exactly where e-mails for the recipient domain name are taken care of. Then, it connects to the remote email server and exchanges information. In case the recipient mail box exists, your server sends the message to the receiving POP/IMAP server and the latter delivers that message to the particular mailbox in which the recipient can get it. If you want to send emails, it is best to ensure that the website hosting company has the service with their packages. Even though you make use of online form that site visitors use to contact you, you'll still require an active SMTP server for your hosting account for the form to function.