SMTP, which is an acronym for Simple Mail Transfer Protocol, is the software that enables you to send out e-mail messages from your email addresses. Without it, you won't be able to employ email applications or webmail. Whenever you send an e-mail, your SMTP server connects to a DNS server to discover where the emails for the recipient domain are taken care of. After that, it creates a connection to the remote mail server and trades details. If the recipient mailbox exists, your server sends the e-mail to the receiving POP/IMAP server and the latter brings that email to the specific mailbox in which the recipient can get it. If you want to be able to send e-mails, you should always make sure that the web hosting company offers the service with their packages. Even when you make use of online form that visitors use to get hold of you, you will still need an active SMTP server for your hosting account for the web form to work.