SMTP, which stands for Simple Mail Transfer Protocol, is the software that allows you to send messages from your email addresses. Without this, you won't be able to use e-mail applications or webmail. When you send an e-mail, your SMTP server connects to a DNS server to find out exactly where emails for the recipient domain name are taken care of. Following that, it creates a connection to the remote email server and trades information. In case the recipient mail box is accessible, your server sends the email to the receiving POP/IMAP server and the latter brings that message to the specific mailbox where the receiver can find it. If you want to send emails, it is best to make sure that the hosting company has the service with their plans. Even if 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.