Interestingly, the path taken by a sent email is not a straight line. Typically, an email does not simply travel from the sender to the recipient, but goes through several intermediate steps as part of a clearly defined process. The key players in this process are the participating SMTP servers, which ensure that the email reaches the recipient in accordance with the protocol. The internet has an entire network of such distribution points and relays to enable the flow of email traffic in the first place.

How does an SMTP server work?

The term ‘SMTP server’ refers to a mail server that forwards emails sent by the sender to one or more recipients via the internet, in accordance with the rules of the network protocol. One of the key functions of an SMTP mail server is to prevent spam by using authentication mechanisms that allow only authorised users to deliver emails. To enable this, most modern mail servers support the ESMTP protocol extension with SMTP-Auth.

So-called ‘relays’ – SMTP servers – are essential links in the email transmission process, which involves several servers: namely, the sender’s outgoing mail server, one or more external relay servers and the recipient’s incoming mail server.

The sender’s outgoing mail server

Once the sender has sent their email, the webmail application or program used to send the email (known as an ‘SMTP client’ or ‘Mail User Agent (MUA)’) formats the message into a header and body before uploading them to the outgoing mail server – the SMTP server. The Mail Transfer Agent (MTA) provides the software foundation for sending and receiving emails. The MTA checks the email for size and spam, then saves it. To lighten the MTA’s workload, it is sometimes preceded by a ‘mail sender agent’, whose task is to carry out a preliminary check on the email’s validity. The MTA then checks the IP address of the recipient’s mail server in the Domain Name System (DNS).

External relay servers

If the recipient’s domain is connected to the same mail server as the sender’s, the email is delivered directly. If not, the MTA splits it into small data packets so that it can be forwarded to the destination SMTP server via the shortest and least congested route.

The recipient’s incoming server

Upon arrival at the destination SMTP server, the data packets are reassembled into a complete email. This is checked again by the MSA and/or the MTA to ensure it does not contain spam, and is then forwarded to the message storage area on the incoming mail server. From there, the ‘Mail Delivery Agent’ (MDA) sends it to the recipient’s email inbox. The email is then downloaded to the recipient’s SMTP client via other network protocols, such as IMAP or POP3.

From a purely technical point of view, it would also be possible to send emails directly from the sender’s SMTP client to the recipient’s SMTP client. However, using an SMTP server offers a clear advantage: if the recipient’s mail server is busy or temporarily out of service, and the email cannot be delivered, the responsible SMTP server will automatically attempt to redeliver the email at regular intervals. This continues until delivery is successful or until the email is returned to the sender as undeliverable.

Which SMTP server should I use?

As a sender, you can choose from several SMTP servers provided by different service providers for feeding emails into and forwarding them via the network. One interesting alternative, which many people opt for, is to set up their own server.

SMTP server from a provider

SMTP servers provided by standard service providers are generally recognised as reliable by other providers. Furthermore, due to the large volume of data processed, their spam filters are considered particularly robust. The main drawback of free SMTP servers is that they generally impose strict limits on the number of emails that can be sent per day, as well as on mailbox size and attachment size.

• Internet service providers: Internet service providers (ISPs), such as IONOS, often provide an internet connection alongside an email address to ensure access to well-known email servers.
• Email providers: Individuals most commonly send emails to friends and relatives via the web-based applications of free email providers, such as Gmail, Yahoo or Outlook. The only requirement is that they have an email address corresponding to a domain that allows the provider’s SMTP server to be used for personal correspondence. You simply need to configure your mailbox to use the correct SMTP server address – below you will find an overview of the most popular providers and their addresses.
• Web hosting providers: Many hosting plans, such as those offered by providers like IONOS, include access to an SMTP server as standard for managing internal and external email traffic.
• Specialist providers: Some companies specialise exclusively in the rental of SMTP servers. Examples include Amazon SES and SparkPost, where you can rent specialised hardware on request.

Personal SMTP server

With some basic IT knowledge, you can set up your own SMTP server. For example, a Raspberry Pi, combined with the appropriate software, provides an excellent hardware platform for this system.

A personal SMTP server has a number of clear advantages: there are no provider-imposed restrictions on usage, you have full control over all settings, and data security is maximised. Furthermore, a self-built server is an ideal way to familiarise yourself with the processes involved in email traffic. Of course, a self-hosted server also has its drawbacks: as a home internet connection uses a dynamic IP address, private SMTP servers are often classified as spam distributors by major email providers and end up in recipients’ junk folders. However, this problem can be overcome with a few configuration changes and/or at an additional cost. All in all, if the sole purpose of your SMTP mail server is to send emails to other private users, a personal server is definitely a great solution.