SMTP, which is short for Simple Mail Transfer Protocol, is the software that allows you to send emails from your email addresses. Without it, you will not be able to use e-mail apps or webmail. Once you send a message, your SMTP server connects to a DNS server to discover exactly where e-mails for the recipient domain are managed. After that, it connects to the remote mail server and trades details. If the recipient mail box is accessible, your server transmits the email to the receiving POP/IMAP server and the latter delivers that email to the particular mailbox where the receiver can find it. If you'd like to be able to send emails, it is best to make sure that the hosting company provides the service with their plans. Even if you use an online form that site visitors use to contact you, you will still require an active SMTP server for your hosting account for the form to function.