SMTP, which is an acronym for Simple Mail Transfer Protocol, is the software that enables you to send messages from your email addresses. Without it, you won't be able to use email apps or webmail. When you send an e-mail, your SMTP server connects to a DNS server to discover where the e-mails for the recipient domain name are taken care of. Following that, it connects to the remote mail server and exchanges details. If the recipient mail box is accessible, your server sends the message to the receiving POP/IMAP server and the latter delivers that message to the particular mailbox in which the receiver can find it. If you want to send e-mails, it is recommended to ensure that the website hosting company provides the service with their plans. Even if 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 web form to work.