SMTP, which stands for Simple Mail Transfer Protocol, is the software that permits you to send out e-mail messages from your email addresses. Without this, you will not be able to use e-mail applications or webmail. When you send a message, your SMTP server connects to a DNS server to learn exactly where emails for the recipient domain are taken care of. Then, it creates a connection to the remote mail server and exchanges information. If the recipient mail box is present, your server sends the message to the receiving POP/IMAP server and the latter delivers that e-mail to the specific mailbox where the receiver can get it. If you want to send e-mails, you should always make sure that the web hosting company has the service with their packages. Even when you make use of online form that site visitors use to make contact with you, you still need an active SMTP server for your hosting account for the form to operate.