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 email applications or webmail. When you send an email, your SMTP server creates a connection to a DNS server to determine where the emails for the recipient domain are taken care of. Subsequently, it creates a connection to the remote email server and exchanges information. 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 where the receiver can access it. If you'd like to be able to send e-mails, it is recommended to make sure that the website hosting company offers 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.