SMTP relay is one service that has been misunderstood and abused not only by ordinary internet users but also by a lot of administrators which is the worst of it all. There are some administrators that are very careless about it leaving it for internet users to do with it as they like. Another group of administrators are too careful that they block it completely. In the light of the above, it is paramount to explain what SMTP relay is. Before explaining what SMTP relay is for a clearer understanding, it will be good that we first explain what SMTP is according to http://spammer.ro/.
The Meaning of SMTP
SMTP is an abbreviation for simple mail transfer protocol. It is widely used today to send emails by many internet service providers. The protocol is utilized by MUA (mail user agents) to transmit emails from the senders to the receivers. The SMTP is the protocol that communicates to SMTP server whenever a MUA sends out email messages. Email messages have similar processes with traditional mails. The SMTP servers perform the function of the post offices. When you send out an email, it first goes to your SMTP server before it is transmitted to the SMTP server of the recipient for delivery to the inbox of the delivery. SMTP therefore works as a message transfer agent (MTA) as it defines the message transmission process.
What is SMTP relay?
SMTP relay is an important service in email delivery. In fact, it will not be possible for emails to transferred from one hosting service to another or from one domain to another if SMTP servers are not able to relay. SMTP relay refers to the service through which email messages are received, sorted and moved from an email hosting server, domains and/or services to another. Basically, it is utilized when emails are to be sent to a domain that is not the same with the domain of the sender. In other words, relays are used for sorting and transmission of emails between two different domains. For example, SMTP relay will be used when an email is being sent from email@example.com to firstname.lastname@example.org.
SMTP relay can be explained with traditional mail delivery process. In traditional mail delivery process, if you send a mail addressed to a recipient in another town to a post office in your town and it accepts the mail and delivers it to another post office in another town, it could be said that your message has been relayed. Similarly, if you send an email to another email id, the email will first of all gets to your server from where it will be delivered to the server of your recipient. Accepting an email by a server to be transmitted to another SMTP server is what is called SMTP relay. In a nutshell, a SMTP relay is a service that mediates the transmission and fetching of mails between one email host and another.
Before SMTP server can permit users to relay mails to the server of the recipient, it will require them to provide their credentials such as username and password. Messages will not be relayed if wrong credentials are provided. However, there are servers that do not require user authentication before they can accept message for another server. This is called open relay.