How an email address works

E-mail transmission: what happens when I send an e-mail?

Sending e-mails is one of the most important Internet services for many, both for business and pleasure. But how does email transfer work from a technical point of view? The standard for forwarding mail is the Simple Mail Transfer Protocol. The messages are retrieved either via IMAP or POP3. In addition, programs such as a mail user agent and mail transfer agent are also used. We tell you what the various software agents are all about, which stations an email goes through during the sending process and which problems can arise during delivery.

This is how email transmission works

At the beginning of every mail dispatch there is the opening of a webmail service via the web browser or an e-mail program installed locally on your computer (also known as an e-mail client). The technical name for an e-mail program is Mail user agent (also Message User Agent or MUA).

After you have written a message in your MUA and clicked the send button, the sending process of an e-mail begins. Because before a mail reaches the addressed inbox, it goes through various stations where it is formatted, checked and forwarded.

  1. Conversion of an email via the MUA

    After you have sent the mail, your e-mail program first converts the message. The MUA divides your mail into two categories - in Header and body. The header area contains information such as the sender, the date and time of sending, the subject of the e-mail or the name of the addressee (s) of the message. The so-called body of a mail follows below the header. The content of an email to be transmitted is referred to as the body. This includes not only the text characters in a message, but also e-mail attachments such as documents, pictures and the like.

  2. Checking an email through an MTA

    Your mail user agent will then contact the Mail server Your e-mail provider (also called e-mail provider). A mail server has a program that is always online to receive and send e-mail: the Mail Transfer Agent (or Message Transfer Agent or simply MTA called). The MTA represents the software basis of a mail server.

    In order to relieve the MTA, a mail or message submission agent (MSA) can be connected upstream. The MSA is in direct contact with the e-mail program. It checks whether the address looks correct and whether it belongs to a real domain. Faulty e-mails are rejected by him, but validated e-mails are forwarded to the MTA. An MSA is often already integrated into an MTA program.

    Before the MTA of your e-mail provider sends the message, it will be directed to the allowable size checked - depending on the e-mail provider, the attachment can usually contain 4 to 20 MB. If the mail or the file attachment is too large, you will be informed and the sending will be stopped. Most e-mail providers also examine the messages for spam and malware such as viruses or Trojans before sending them. If there is no suspicion of spam and the mail size is within the permitted range, the MTA saves the message.

  3. The sending process up to the receiving MTA

    In the next step Your Mail Transfer Agent searches for the recipient's mail server in the Domain Name System - if this has not already been done by the MSA. If the corresponding server has been identified, the next step is to check whether the local part of the address - i.e. everything that precedes the @ sign - also exists. If the e-mail address cannot be found (because you made a mistake or it simply does not exist), the mail is sent back to the sender by the MTA with a corresponding note. If everything is correct, the own MTA forwards the mail to the MTA of the recipient mail server.

    As with any data traffic on the Internet, the message is broken down into several parts. Sending individual packages (their maximum size is 64 KB) has several advantages: On the one hand smaller broadcasts can be transmitted more easily. On the other hand, this makes it possible for the individual parts of an email use different data pathsto reach the destination mail server. The packets always fall back on the transmission paths on which there is currently relatively little traffic.

    The data traffic between mail servers is based on the use of Internet node. Countless amounts of data are transmitted through these nodes. They are an essential part of the internet infrastructure. This is where email providers exchange messages with one another. The individual packets of a mail reach the recipient's mail server via these distribution nodes and are reassembled there.

  4. Second check at the destination MTA and delivery of the mail

    The mail has now arrived on the recipient's mail server. But before it reaches the addressee, also checks the MTA of the destination mail server the incoming message. Again, the size of the attachments is checked: If the file attachment exceeds the capacity of the recipient's mail provider, the message is not delivered but sent back to you. In addition, spam content and malware are searched for again. If suspicious signs of this are found (e.g. certain terms such as "Viagra" in the text, known viruses or other computer malware in the attachment - or if the sender in question has already sent spam frequently), the e-mail is marked accordingly or the transfer of the message is completely prevented.

    If your mail has also successfully passed this check, the Message saved on the recipient's mail server and can be accessed by the addressee. For this purpose, a mail or message delivery agent (MDA) sends the message to the recipient's e-mail inbox. As soon as the recipient calls up his or her e-mails, the MUA used accesses the MTA of the mail server. The content is checked again for spam and malware - this time by the recipient's email program.

    The mail can then be opened and read in the inbox. The many intermediate steps and security measures are intended to ensure that as few unwanted or harmful emails as possible reach the digital mailboxes. All of these processes that an email goes through until it reaches the recipient only take a few seconds.
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