SMTP, which stands for Simple Mail Transfer Protocol, is the software that allows you to send out messages from your email addresses. Without it, you will not be able to employ email applications or webmail. When you send an e-mail, your SMTP server creates a connection to a DNS server to determine where the emails for the recipient domain name are taken care of. Following that, it creates a connection to the remote mail server and trades details. In case the recipient mailbox is available, your server sends the message to the receiving POP/IMAP server and the latter brings that e-mail to the specific mailbox in which the recipient can find it. If you want to send emails, it is best to ensure that the website hosting company provides the service with their plans. Even if you make use of online form that website visitors use to contact you, you'll still require an active SMTP server for your hosting account for the form to work.