SMTP, which is an acronym for Simple Mail Transfer Protocol, is the software that enables you to send out emails from your email addresses. Without this, 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 find out where the emails for the recipient domain name are taken care of. Then, it connects to the remote email server and exchanges details. In case the recipient mailbox exists, your server transmits the email to the receiving POP/IMAP server and the latter delivers that message to the particular mailbox in which the receiver can see it. If you would like to send e-mails, it is recommended to ensure that the hosting company has the service with their plans. Even if you work with an online form that site visitors use to contact you, you'll still need an active SMTP server for your hosting account for the web form to work.