Interestingly, the path of the email sent is not a straight line. Typically, an email does not simply travel from sender to recipient, but goes through a series of intermediate steps in a clearly ordered process. The main actors in this process are the participating SMTP servers, which ensure that the e-mail reaches the recipient according to the protocol. The Internet has a whole network of such hubs and relays to allow the movement of e-mail traffic at all.
How does an SMTP server work?
An “SMTP server” refers to a mail server that forwards e-mails sent by a sender to one or more recipients over the Internet according to the rules of a network protocol. An important function 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.
The so-called “relays” – SMTP servers – are the basic links in the email forwarding process involving several servers: namely the sender’s outgoing mail server, one or more external relay servers and the recipient’s incoming mail server.
Sender’s outgoing mail server
Once the sender has sent his/her email, the webmail application or program used to send the email (called the “SMTP client” or “Mail User Agent (MUA)”) converts the message into a header and a deletion before uploading them to the outgoing mail server – the SMTP server. The server Mail Transfer Agent (MTA) is the software basis for sending and receiving emails. The MTA checks the email for size and spam and saves it. To reduce the burden on the MTA, it is sometimes preceded by a “sending agent”, which is responsible for checking the validity of the email in advance. The MTA then checks the IP address of the recipient’s mail server in the Domain Name System (DNS).
External forwarding servers
If the recipient domain is connected to the same mail server as the sender, the e-mail is delivered directly. If not, the MTA will split it into small data packets to forward it to the destination SMTP server via the shortest and least congested route possible.
Incoming server of the recipient
On arrival at the destination SMTP server, the data packets are reassembled into a complete email. It is then checked again by the MSA and/or MTA for spam and sent to the message storage area on the incoming mail server. From there, it is sent by the “Mail Delivery Agent” (MDA) to the recipient’s mailbox. Other network protocols – IMAP or POP3 – then download the email to the recipient’s SMTP client.
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, there is a clear advantage to using an SMTP server: if the recipient’s mailbox server is busy or temporarily down and the email cannot be delivered, the responsible SMTP server will automatically try to redeliver the email at regular intervals. This will continue until the 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 from different providers to load and forward emails to your network. One interesting alternative that many people choose is to set up their own server.
SMTP server from a service provider
SMTP servers from standard providers are generally recognised as trustworthy by other providers. Furthermore, due to the large amount of data processed, their spam filters are considered particularly strong. The main disadvantage of free SMTP servers is that they usually have strict limits on the number of emails that can be sent per day, the size of the mailbox and the size of attachments.
- ISPs: Internet Service Providers (ISPs), such as IONOS, often provide an Internet connection along with the e-mail address to give access to big-name e-mail servers
- E-mail service providers: individuals most often send e-mails to friends and relatives via a free e-mail service provider’s web application, such as Gmail, Yahoo or Outlook. The only requirement is that they have an email address corresponding to a domain that can be used to send personal correspondence using the provider’s SMTP server. All you need to do is set your mailbox to the address of the appropriate STMP server – below is an overview of the most popular providers and their addresses.
- Hosting providers: many hosting provider packages, such as those from providers like IONOS, include access to an SMTP server as standard to manage internal and external mail traffic.
- Specialist providers: some companies specialise in renting SMTP servers exclusively. For example, Amazon SES and SparkPost, where you can rent specific hardware on request.
For example, some companies such as SparkSoft and PostServer.com offer specialised SMTP servers.
Personal SMTP server
With some basic IT knowledge, you can set up your own SMTP server. For example, a Raspberry Pi computer with the right software provides an excellent hardware basis for this system.
A personal SMTP server has some clear advantages: no service provider restrictions on usage, full control over all settings, and maximum data security. Also, a self-built server is an ideal way to get to know the processes around email traffic. Of course, having your own server also has its drawbacks: since the personal Internet broadcasts a dynamic IP address, private SMTP servers are often classified as spam distributors by the big mail providers and end up in the recipients’ trash can. However, this problem can be overcome with some redesign measures and/or additional costs. All in all, if your SMTP mail server is only used to send emails to other private clients, a personal server is a great solution.
