SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol, basically a set of rules and guidelines that a system has to follow, which is used in sending and receiving information in the form of electronic mail.[1] However, it is actually restricted and lacks a few features, such as the ability to queue messages at the receiving end since it is usually used by either IMAP (Internet Message Access Protocol) or POP3 (Post Office Protocol 3). These protocols allow the user to access their messages and their content in a mailbox that is saved within the server, while the system periodically downloads them from the server’s database.

On other operating systems, such as Unix, Sendmail is the most used SMTP server for emails.[1:1] These protocols are also vital when it comes to administrating, managing and securing any network activity. Fortunately, for the average email service user, they’re already built-in to the system so that users won’t have to deal with the hassle of managing a network manually. However, in rare and unusual instances, clients will need to become familiarized with protocols, such as IP (internet protocol) and SMTPs. SMTP has a major influence in every single email that an individual has sent throughout their whole lifetime.

Usual email service providers, such as Yahoo mail and Gmail, have their own associated SMTP server or host which is dedicated towards the process of delivering the email. Users and experts alike, however, need to be aware that the highest quality of professional SMTP service like turboSMTP ensures the highest rate of delivery and transmission.[2]


SMTP is the most common and widely used protocol for email communication over the internet. It provides a medium for network services between the email provider or, in some cases, the organizational email server, and the user that is accessing the email.[1:2] It usually uses a process that’s called “Store and forward”. This moves emails across networks. In this case, it would closely work together with a major component that is called the Main Transfer Agent (MTA) which users have to use to send communications to the right computer and email inbox. After which, SMTP will then spell out, while also directing how an email will be transmitted from the local computer’s MTA to another MTA on another device, and even numerous other computers or devices.[3]

Simply put, when users send a message to a friend, known as the recipient, using a mail client or service, it will be picked up by a program called the outgoing server which would then initiate a conversation between the sender and receiver that will also incoming. During this process, the machines communicate with each other through SMTP. This can be likened to a specific language that both machines are fluent in. Since SMTP is a protocol, it provides a set of guidelines that both machines have to adhere to. In this way, both of the servers will be able to identify themselves and each other and communicate so that they will be able to recognize (a) the sender, (b) where the message should be going, (c) where the content must go and a multitude of different functions.[4] Basically, SMTP shows the server and the system how to properly deliver the email while also solving issues.

SMTP also sometimes stands for ‘stop’. Most users don’t know this, but Internet Service Providers have a limit when it comes to the number of emails that we can send out at a certain point of time.[5] In some instances, it’s limited to every hour or every day. The reason why this was done is to prevent spam or unwanted emails from being sent from automated email servers that have been programmed to keep on sending messages to users and businesses.

Each Internet Service Provider relies on its own SMTP to manage, administrate, and govern the email that is sent out from its source device. For users who work at home or those who manage large quantities of email lists, this could prove to be detrimental. This is due to the fact that if the SMTP hits its limit, the Internet Service Provider will simply stop sending the email. In some cases, if the system is automated and left without supervision, the system might assume that the user is a spammer from sending large quantities of email and will shut down the account.[5:1]

However, most users don’t have to worry about this protocol since the email limit varies depending on the ISP. As an example, your average customer, who avails a Comcast Cable program is limited to only being able to sned out 1,000 a day while for a different plan, targeted for businesses, has a 24,000 limit per day. Several other internet service providers like Verizon and AT&T have a more non-conventional approach. Instead, both these ISPs put a limit of a 100 on a number of recipients that users are allowed to send from one email.


SMTP is generally built-in with an email client and is usually composed of the following components:[3:1]

  • Local User / Client-end utility (Mail User-agent)
  • Mail Submission Agent (MSA)
  • Mail Transfer Agent (MTA)
  • Mail Delivery Agent (MDA)


Despite all of these features, SMTPs are restricted to only be able to transfer text content. This means that it is unable to recognize fonts, attachments, images, video, audio files, etc. Which is one of the reasons why this is called "simple".[3:2] Because of the rapidly evolving changes in technology and the internet, MIME (Multipurpose Internet Mail Extensions) was created to fill in the technological gap. MIME is able to encode and transmit all of the necessary information that SMTP is not able to do. This means that MIME is able to transfer images, attachments, documents, audios, videos, and other types of files and also encodes all of these content into plain text. Once these files and data have been translated into this format, the SMTP will now be able to transfer data as necessary.

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