SMTP, which is short for Simple Mail Transfer Protocol, is the software that enables you to send e-mail messages from your email addresses. Without this, you won't be able to use email applications or webmail. When you send an email, your SMTP server connects to a DNS server to determine exactly where e-mails for the recipient domain are taken care of. Then, it connects to the remote mail server and trades details. When the recipient mail box is accessible, your server transmits the e-mail to the receiving POP/IMAP server and the latter brings that e-mail to the specific mailbox where the receiver can get it. If you want to be able to send emails, you should always ensure that the hosting company has the service with their plans. Even if you use an online form that website visitors use to contact you, you'll still need an active SMTP server for your hosting account for the form to operate.