SMTP, which is an acronym for Simple Mail Transfer Protocol, is the software that allows you to send out e-mails from your email addresses. Without it, you will not be able to use email apps or webmail. After you send an email, your SMTP server connects to a DNS server to discover where the e-mails for the recipient domain are taken care of. Then, it connects to the remote mail server and trades details. When the recipient mailbox is accessible, your server sends the e-mail to the receiving POP/IMAP server and the latter brings that e-mail to the precise mailbox where the recipient can find it. If you'd like to be able to send emails, it is best to make sure that the hosting company offers the service with their packages. Even if you make use of online form that visitors use to contact you, you'll still require an active SMTP server for your hosting account for the form to function.