What is a Mail Server - its tasks

Communication via email is considered standard in many areas today and has long taken over the importance of traditional mail correspondence. The mail server is responsible for receiving, sending, storing, and retrieving emails, and is an essential component of a server setup. Communication always takes place via the Simple Mail Transfer Protocol (SMTP), which defines a standard for sending, forwarding, and receiving electronic messages. On the other hand, managing mail by the owner of an address from an external computer is done through the Post Office Protocol (POP3) or the Internet Message Access Protocol (IMAP). Both protocols have their own characteristics and features, so a comparison of IMAP vs. POP3 should be made in each situation.

Sending and receiving messages - How a mail server works

Users typically do not directly interact with the actual mail server, as it manages the connection between individual computers in a network. It requires two essential pieces of information, which are integral parts of every email address:

• A fixed destination address such as a domain, an IP address, or a hostname like localhost
• A user at this destination address to whom the mail server delivers the message

Both pieces of information are communicated to the mail server in the familiar format user@destination address, providing all the necessary parameters for allocation. The exact process of sending an email involves the following steps:

• A user writes an email using a web interface or an email program like Thunderbird or Microsoft Outlook
• When clicking Send, the software hands over the message to the mail server
• The sending mail server queries the nameservers for a route to the recipient
• It then contacts the first instance, identifies itself, and provides the sender and recipient addresses
• If the other mail server accepts the request, the message is transmitted in text form
• If the endpoint is not reached, it contacts the next location
• At the destination, the mail server receives the message and stores it locally
• The recipient can then retrieve and read the email using IMAP or POP3

One peculiarity of SMTP is that the protocol originally only provided for the transmission of texts in ASCII code. Therefore, binary data such as attachments are encoded by the server for transport as texts and converted back to their original format at the destination.

Limitations of SMTP Security: No Encryption or Authentication

Only a few users are aware that their emails are passed from one mail server to another completely readable, in plain text and without any encryption. Every node involved in the transport can read their content without restrictions. Furthermore, the protocol does not require any sender authentication, allowing a user to specify any address as the origin with minimal effort. The reason for this is that the protocol was designed and planned as a local messaging system when it was introduced in the mid-80s of the last century.

In order to securely identify contacts and make the content unreadable to foreign servers, additional local encryption by the user is therefore necessary. This is done through a method called asymmetric cryptography, where encryption is done with a public key, but the content can only be made readable again with another key. This ensures that only the recipient, as the holder of the corresponding private and secret key, has access to a message.

Reading Messages and Other Data - IMAP vs. POP3 vs. Hosted Exchange

For users to read their messages, they connect to a client with the mail server to have the mail delivered to them. There are two different protocols, each with their own advantages and disadvantages. With POP3, it is a classic protocol for transmitting information, delivering emails and their content to the client. On the other hand, IMAP is strictly speaking a file system designed specifically for emails, allowing the client remote access to storage on the remote server. It enables a much more extensive configuration, hierarchical structuring of folders, and shared access by multiple users with individual rights to an email address. Hosted Exchange is a Software-as-a-Service (SaaS) offered by Microsoft that enhances the functionality of emails. For example, synchronising contacts and calendars across multiple devices is possible. It is offered by numerous providers as a complement to traditional email and for deeper integration in environments using Microsoft Windows as the operating system.

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