What is an SMTP relay and how does it work?

An SMTP relay is the for­ward­ing of emails from senders to re­cip­i­ents. The SMTP relay server acts as an in­ter­me­di­ary to deliver emails reliably and securely, es­pe­cial­ly when they are sent across different servers or networks. This ensures messages are correctly delivered and not blocked as spam.

What is an SMTP relay?

The term “SMTP relay” (or “email relay”) is defined quite in­con­sis­tent­ly on the internet. Some sources refer to it as a process, others as a service, and still others use the word syn­ony­mous­ly with SMTP server or even lump it together with the Simple Mail Transfer Protocol (SMTP). In a way, they are all right but fail to ad­e­quate­ly integrate the in­di­vid­ual aspects.

Here is a clear de­f­i­n­i­tion: An SMTP relay is the process of for­ward­ing emails across the internet from senders to re­cip­i­ents. This process is handled by SMTP relay servers, which operate according to the Simple Mail Transfer Protocol, and can be part of an external service (an SMTP relay service).

What is an SMTP relay server?

An SMTP relay server (also known as a mail relay server or mail transfer agent) is, based on the previous de­f­i­n­i­tion, an SMTP server that enables an SMTP relay. This means it accepts emails from the sender and forwards them to third parties. Typically, it is an external third-party server on the internet, po­si­tioned between the sender and recipient SMTP servers (along with any ad­di­tion­al mail transfer agents, or MTAs). When sending messages within the same mail provider domain, no ad­di­tion­al server is involved, so it is not con­sid­ered an email relay in those cases.

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The dif­fer­ence can also be il­lus­trat­ed: If you send a letter to a distant relative, it is first relayed by the local post office to the one nearest the recipient before being delivered (relay); however, when sent within the same town, the local post office delivers the letter directly to the des­ti­na­tion address (no relay).

An SMTP relay server can be either “closed” or “open” based on its con­fig­u­ra­tion. In this context, we dis­tin­guish between a “smarthost” and an “open mail relay”. Ad­di­tion­al­ly, an email relay placed in front of the sender’s outbound server can perform other useful functions.

Smarthost

A properly con­fig­ured SMTP relay server is also known as a “smarthost.” It is “smart” in the sense that it only forwards an email if it is re­spon­si­ble for either the sender or the recipient. Emails from unau­tho­rized parties outside the server’s ju­ris­dic­tion are rejected. This is enabled through various au­then­ti­ca­tion methods such as whitelists, SMTP-after-POP, or ESMTP with the SMTP Auth extension.

Open mail relay

A dis­ad­van­tage of SMTP is that its basic con­fig­u­ra­tion does not require user au­then­ti­ca­tion. If you don’t manually add the ap­pro­pri­ate mech­a­nisms to an SMTP relay server, it is con­sid­ered poorly con­fig­ured and thus an “open mail relay.” In plain terms, this means the server will also forward emails for which it is re­spon­si­ble neither for the sending nor the receiving person.

This situation allows cy­ber­crim­i­nals to dis­trib­ute massive amounts of spam on the internet using fake or stolen sender addresses. If such an open relay is dis­cov­ered, it usually lands on the black­lists of major webmail providers within hours or days. All messages sent through it, even le­git­i­mate ones, are then clas­si­fied as spam and filtered out. As the operator of the in­cor­rect­ly con­fig­ured SMTP relay server, you face the con­se­quences: you must fix the security flaw and request removal from the blacklist. For busi­ness­es and or­ga­ni­za­tions, this extra work can lead to economic losses.

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Uses of SMTP relay servers

Ex­ten­sions and upgrades of email systems are usually not im­ple­ment­ed directly on the sender’s outgoing mail server, but rather through a front-end SMTP relay server. To “retrofit” several functions at once, complete “relay chains” are possible, which process an email in various ways before it is fed into the internet. These ad­di­tion­al areas of ap­pli­ca­tion include, among others:

• Combating spam with senders: To maintain their online rep­u­ta­tion, it is also important for operators of outgoing mail servers to prevent the sending of spam, viruses, and other malware by their own users. Ap­pro­pri­ate scanners and firewalls are usually installed on a separate SMTP relay server to offload the actual mail server.
• Backup mail service for re­cip­i­ents: A smarthost can also accept emails for the recipient’s incoming mail server when it is tem­porar­i­ly un­avail­able for any reason. Once it is back online, the smarthost gradually delivers the ac­cu­mu­lat­ed emails.

Further possible ap­pli­ca­tions of SMTP relay servers include:

• the signing and en­cryp­tion of emails
• the automatic addition of a dis­claimer at the end of each message text
• the archiving of emails
• mail routing and rewriting for address changes
• load balancing and scal­a­bil­i­ty

What is an SMTP relay service?

To coun­ter­act the daily flood of spam, most internet and email providers limit the number of messages that can be sent per day. Bulk emails, newslet­ters, and automatic trans­ac­tion­al messages like order con­fir­ma­tions, which are to be sent to thousands of re­cip­i­ents, require an af­ford­able, scalable, and reliable delivery platform capable of meeting the specific needs of a growing business.

One option is to set up your own SMTP relay server. However, using your own hardware for large volumes of external business emails can result in bandwidth lim­i­ta­tions and hinder internal peer-to-peer cor­re­spon­dence. Another dis­ad­van­tage is the cost and sig­nif­i­cant workload as­so­ci­at­ed with server ad­min­is­tra­tion. Ad­di­tion­al­ly, emails from foreign SMTP relay servers are often clas­si­fied as spam, as are those with dynamic IP addresses.

Due to these re­stric­tions, many companies prefer to turn to a pro­fes­sion­al SMTP relay service, which operates its own server for email for­ward­ing. Mailing packages from providers like mailjet, Brevo, or turboSMTP are spe­cial­ized in handling large volumes of emails, scalable in their pricing model based on the volume of messages, and typically maintain an excellent rep­u­ta­tion with internet and mail providers due to their close col­lab­o­ra­tion. Once you have set up the re­spec­tive service, you can continue to use your email ap­pli­ca­tion (e.g., Microsoft Outlook) as usual to send and receive messages.

Ad­di­tion­al­ly, many SMTP relay services offer added value, such as a reporting feature: By having the provider’s SMTP relay server au­to­mat­i­cal­ly insert a link tracker into the body of each email, you can collect eco­nom­i­cal­ly valuable data such as open, click, and bounce rates, as well as feedback on the sending process or your newslet­ter (e.g., spam com­plaints or sub­scrip­tion can­ce­la­tions). When properly in­ter­pret­ed and utilized, this in­for­ma­tion can serve as a basis for adjusting and modifying your email marketing campaigns.

How to use the IONOS SMTP relay

To use the IONOS SMTP relay, an email mailbox must be set up as a relay address belonging to the same domain as the cor­re­spond­ing SPF entry. Specif­i­cal­ly, this means that the email addresses using the SMTP relay server must also belong to the domain of the relay address. You can easily create an SPF entry for your domain through the IONOS Help Center to ensure suc­cess­ful email delivery. These pre­req­ui­sites ensure a secure and reliable use of the SMTP relay within your domain.