SMTP, which stands for Simple Mail Transfer Protocol, is the software that allows you to send emails from your email addresses. Without it, you won't be able to use email apps or webmail. Once you send a message, your SMTP server connects to a DNS server to discover where the emails for the recipient domain are taken care of. Following that, it creates a connection to the remote mail server and exchanges information. When the recipient mailbox is present, your server transmits the e-mail to the receiving POP/IMAP server and the latter delivers that e-mail to the specific mailbox where the receiver can see it. If you wish to be able to send e-mails, you should always make sure that the website hosting company provides the service with their packages. Even when you use an online form that visitors use to get hold of you, you still require an active SMTP server for your hosting account for the form to function.