An SMTP relay server, also called a smarthost, is an intermediary SMTP server that receives email from a company, application or local server and forwards it to the recipients’ servers. Its function is to centralize and manage outgoing email in order to simplify the infrastructure, apply security controls and improve deliverability.
Mailrelay can be used as an outgoing SMTP server and as a smarthost for Exchange, Postfix and other servers or applications that support SMTP. This way, the organization keeps its usual tools while delegating the processing of outgoing email to a specialized infrastructure.
Quick summary
- SMTP server: a system that sends, receives or relays email using the SMTP protocol.
- SMTP relay: a function that accepts a message from an authorized system and forwards it to another mail server.
- Smarthost: an SMTP relay server configured as the central outbound route for one or more systems.
- Main advantage: it lets you manage outgoing email from a single point, without configuring direct delivery on every machine or application.
- Mailrelay as a smarthost: it receives messages from authorized systems, processes them and delivers them to the destination servers.
What is an SMTP server?
An SMTP server is a system that uses the Simple Mail Transfer Protocol to send and transfer email messages. It can communicate with applications, email clients and other SMTP servers.
In a typical business sending flow, the outgoing SMTP server receives the message, checks that the sender is authorized, places it in a queue and tries to deliver it to the server that handles the recipient’s domain. If delivery is not immediately possible, it can try again or generate a bounce with information about the error.
SMTP is used for outgoing email. Checking and downloading received messages is usually done through other protocols, such as IMAP or POP3.
What is an SMTP relay?
An SMTP relay is the process by which an SMTP server accepts email coming from an authorized client, application or server and relays it to the next server along the route.
Instead of each system delivering messages directly to the servers of Gmail, Outlook, Yahoo or other providers, all outgoing email is sent to the SMTP relay first. The relay takes care of queuing, routing, retries and communication with the destination servers.
The term is also used to describe the service that provides this function. That is why it is common to speak interchangeably of an SMTP relay service, a mail relay or an SMTP relay server.
What is a smarthost?
A smarthost is an SMTP relay server defined as the outbound route for an organization’s email. It acts as an intermediate point between the sender’s infrastructure and the recipients’ servers.
Its purpose is to prevent every server, device or application from having to handle direct email delivery on its own. The smarthost concentrates that task and makes it possible to apply a common policy for authentication, security, reputation and tracking.
In practice, the terms smarthost and SMTP relay usually refer to the same solution. “SMTP relay” describes the relaying function; “smarthost” emphasizes the central server configured to perform it.
Differences between an SMTP server, an SMTP relay and a smarthost
| Term | Meaning | Typical use |
|---|---|---|
| SMTP server | System that transfers email using SMTP | Sending messages between applications and servers |
| SMTP relay | Function or service that relays authorized email | Delegating delivery to an external infrastructure |
| Smarthost | SMTP relay server used as the central outbound route | Centralizing the email of a company, server or network |
A smarthost is therefore a type of SMTP server that offers SMTP relay functions. Not all SMTP servers are configured to accept and relay messages from third parties.
How a smarthost works, step by step
The basic flow of sending through an SMTP relay server is as follows:
- A user, CRM, ERP, website, printer or mail server generates a message.
- The source system sends the message to the smarthost via SMTP.
- The smarthost verifies that the source is authorized, for example through SMTP credentials or an allowed IP address.
- The SMTP server processes the message, applies the configured rules and adds it to the sending queue.
- The SMTP relay looks up the destination and delivers the email to the corresponding server.
- The system records the result: delivery accepted, attempt pending, rejection or bounce.
The route can be summarized as follows:
Application or local server → smarthost / SMTP relay → recipient’s server → inbox
What is an SMTP relay server used for?
An SMTP relay server is useful when an organization needs a centralized, scalable and controlled outbound email route. Its most common applications include the following.
Centralizing outgoing email
A company can configure Exchange, Postfix or another local server to send all its email through a single smarthost. This way it avoids maintaining different outbound routes and delivery policies.
Connecting business applications
CRMs, ERPs, e-commerce platforms and internal tools can use a common SMTP server to send notices, confirmations, invoices, password changes and other operational communications.
Sending notifications from devices
Printers, monitoring systems, PBX systems, cameras, NAS devices and other equipment can generate email alerts. An SMTP relay makes it possible to channel these messages even when the devices have limited configuration options.
Reducing dependence on the local IP
Direct delivery from a corporate IP requires managing reputation, blocklists, reverse DNS and other technical aspects. A specialized SMTP server provider can take on much of that work. This does not by itself guarantee that messages will reach the inbox: list quality, consent, content and domain authentication remain decisive.
Separating message generation from delivery
The application takes care of creating the email and handing it to the smarthost. The SMTP relay server manages queues, retries and communication with the destination domains. This separation makes it easier to maintain and grow the infrastructure.
Advantages of using Mailrelay as a smarthost
Mailrelay lets you use its infrastructure as a traditional SMTP server and as a smarthost for Exchange and other mail servers.
Centralized configuration
Instead of modifying every account or machine, you can configure the SMTP relay server as the outbound route on the central server. Applications and users keep sending from their usual environment.
Infrastructure specialized in email sending
Delegating outgoing email lets you take advantage of a platform built to process messages, manage queues and communicate with many destination providers. The specific capacity and terms depend on the plan you have purchased.
Domain authentication
Using SPF and DKIM correctly helps email providers verify that the email is being sent with authorization. DMARC lets you define a policy for messages that fail those checks and receive reports on the use of your domain.
Results tracking
When the service and the type of message allow it, statistics make it easier to analyze deliveries, bounces, opens and clicks. Opens are not an exact measure—they can be affected by privacy mechanisms—so they are best interpreted alongside other metrics.
Compatibility with existing tools
A smarthost based on SMTP standards can be integrated with mail servers and many applications without rebuilding the system that generates the messages.
Operational support
Access to specialized support can simplify the initial setup, the diagnosis of rejections and the review of authentication or deliverability issues. You should check the current terms of the service to learn the scope of support and the hours it is available.
Use cases for an SMTP relay server
1. Hosting providers and ISPs
A provider with reputation problems or whose IPs are listed on blocklists can configure a smarthost as its outbound route. This way it centralizes delivery without requiring individual changes for all of its customers, as long as the service supports this model and abuse controls are applied.
2. Companies with Exchange, Postfix or another local server
An organization can keep its internal server and use Mailrelay as its outgoing SMTP server. Mailboxes hand the email to the corporate system, which relays it to the smarthost.
3. CRMs, ERPs and in-house applications
Applications can send transactional or operational messages via SMTP without implementing a full delivery engine. It is important to separate transactional traffic from promotional traffic where appropriate, and to respect the policies that apply to each type of communication.
4. Technical alerts and devices
Help desk services, monitoring systems and printers can use the SMTP relay to send incidents, status notices and internal notifications from a common outbound route.
5. Organizations with multiple offices or senders
A smarthost lets you apply common security and routing criteria to different departments, domains or locations. Segmenting by sender makes control and diagnosis easier.
Example: using a smarthost in a company
Suppose a company uses Microsoft Exchange to manage hundreds of corporate accounts. Instead of delivering each email directly from its own connection, it configures an outbound connector to the SMTP relay server.
From then on:
- employees keep sending messages from their usual clients;
- Exchange forwards outgoing email to the smarthost;
- the SMTP relay processes and delivers the messages;
- the technical team can review the results from a central point.
Mailrelay documents a use case of this kind with Tata Motors, where the smarthost service was used as the SMTP platform for more than 300 corporate accounts. The case should be read in its context and not as a guarantee of identical results for every organization.
How to set up an SMTP relay server or smarthost
The exact details depend on the provider, the plan and the source system, but a typical setup requires:
- SMTP server: the hostname provided by the provider.
- Port: the one specified for the available connection.
- Encryption: TLS or STARTTLS, according to the service documentation.
- Authorization: SMTP username and password, or an allowlist of IP addresses.
- Senders and domains: identities authorized to send.
- DNS records: SPF, DKIM and, ideally, DMARC.
- Sending limits: supported volume, speed and number of connections.
After saving the configuration, run a controlled test and verify:
- that the connection to the SMTP server is established correctly;
- that the message passes the SPF and DKIM checks;
- that the sender and the domain are authorized;
- that bounces are processed as expected;
- that you have not created an open relay that unauthorized senders can use;
- that the logs and statistics show the result of each send.
To avoid errors, always check Mailrelay’s current parameters and the specific documentation for Exchange, Postfix or the application you use.
Security and best practices
An SMTP relay must accept messages only from authorized sources. A server open to any sender can be used to send spam and quickly damage its reputation.
These are the main recommended measures:
- use encrypted connections when available;
- protect and rotate SMTP credentials;
- restrict access by IP when the scenario allows it;
- authorize only the necessary domains and senders;
- configure SPF, DKIM and DMARC;
- separate transactional and promotional flows when appropriate;
- monitor volume spikes, bounces and anomalous patterns;
- maintain consent-based recipient lists and clear unsubscribe processes;
- comply with the applicable privacy and commercial communications regulations.
Common problems when using a smarthost
The SMTP server rejects the connection
Check the server name, the port, the encryption method, the credentials and the firewall rules. If authorization is IP-based, confirm that the outbound public IP matches the registered one.
The sender is not authorized
Verify that the sender address or domain is enabled and that the credentials belong to the right account. Also review the alignment between the visible domain, DKIM and the DMARC policy.
Messages are accepted but not delivered
Check the logs and the response codes from the destination server. Causes can include nonexistent addresses, reputation, content, temporary limits, anti-spam policies or domain authentication errors.
Emails end up in spam
An SMTP relay server improves technical control, but it does not replace good practices. Review contact consent, list quality, frequency, content, links, authentication and domain reputation.
Frequently asked questions about SMTP relay and smarthost
Are SMTP relay and smarthost the same thing?
In many contexts they are used as synonyms. SMTP relay describes the relaying of email between servers; smarthost refers to the intermediary server configured as the central outbound route.
What is the difference between an SMTP server and a smarthost?
SMTP server is the general concept. A smarthost is an SMTP server configured to receive email from authorized systems and relay it to the destination servers.
Does a smarthost replace Exchange or Postfix?
Not necessarily. Exchange or Postfix can keep managing internal email and hand outgoing messages to the smarthost. The SMTP relay server then handles the next stage of delivery.
Is authentication needed to use an SMTP relay?
Yes, the source must be authorized. Depending on the service and the scenario, authorization can be done with SMTP credentials, an allowlist of IPs or other controls. An open relay must not be configured.
Does an SMTP relay server guarantee that emails reach the inbox?
No. It can provide better infrastructure and better control over delivery, but the result also depends on domain reputation, authentication, consent, list quality, content and the receiving provider’s policies.
Can I use Mailrelay as an outgoing SMTP server?
Yes. Mailrelay states that its service can be used as a traditional SMTP server and as a smarthost for Exchange and other mail servers. Availability, parameters and specific limits should be confirmed in your account or with the Mailrelay team.
Which systems can connect to a smarthost?
In general, any server, application or device that supports SMTP and is supported by the provider’s configuration: Exchange, Postfix, CRMs, ERPs, online stores, internal tools, monitoring systems and printers, among others.
Conclusion
An SMTP relay server or smarthost centralizes outgoing email and delegates its delivery to a specialized infrastructure. It is an especially useful solution for companies with their own servers, business applications, multiple senders or devices that need to send notifications.
Mailrelay can act as an outgoing SMTP server so that the organization keeps its current tools and routes messages through a single point. Before implementing it, you should define the volume, the types of email, the authorization method, the domain authentication and the metrics you need.
If you need to know whether this architecture fits your environment, contact Mailrelay and tell us which system you use, how many senders will be sending and what your estimated volume is.
Application example and success story
Here we can see a real practical example of applying the Mailrelay smarthost service, along with the success story achieved with our client Tata Motors:
Portfolio ⇒ TATA Motors
If you would like more information about how to use Mailrelay as a smarthost server, just get in touch with us using this form.
A good setup of your sending server directly affects deliverability in email marketing, and in some cases it is worth relying on sending domain rotation to protect your reputation.