In many companies, \u201csending email\u201d sounds like a single technical function. In reality, there are two distinct sending models with different security assumptions: user sending from an email client (SMTP submission) and server-to-server transfer (SMTP relay). Because both involve SMTP and often the same mail server software, the terms get used interchangeably. That confusion can lead to misconfigurations, access problems, deliverability issues, and in the worst cases, serious abuse such as open relay.<\/p>\n
This article explains what SMTP relay is, how it differs from SMTP submission, which one fits common business scenarios, and what \u201csecure by design\u201d looks like when you run email for a company.<\/p>\n
It\u2019s easier to make correct choices when you know the roles involved in email sending:<\/p>\n
A single platform can provide more than one of these roles. The important part is how you separate traffic types (users vs systems vs server-to-server) and apply the right controls to each path.<\/p>\n
SMTP submission<\/strong> is the method used when a user (or a user-facing app acting on behalf of a user) sends email through the company mail system. The submission path is designed around identity: the sender proves who they are, the server enforces policies, and the connection is typically protected with TLS.<\/p>\n Common characteristics of SMTP submission include:<\/p>\n In short, submission is the secure \u201cclient-to-server\u201d doorway for outbound mail.<\/p>\n SMTP relay<\/strong> refers to a server passing email to another server. This is the normal behavior of email on the internet: MTAs relay messages toward the recipient domain. Inside a business, \u201crelay\u201d also describes application and device sending via a designated relay host (for example, a web app, a monitoring system, or a printer submitting mail to an internal relay server).<\/p>\n Typical relay characteristics:<\/p>\n Relay is fundamentally \u201cserver-to-server\u201d transport, and its authorization model must be designed carefully.<\/p>\n Submission is identity-driven: users authenticate, and you can apply per-user rules. Relay is often network-driven: only approved systems or networks are allowed to relay. Both can be secure, but they fail in different ways. For example, stolen credentials are a submission risk; overly broad network allowances are a relay risk.<\/p>\n Well-run environments separate submission traffic from relay\/transfer traffic so policies are clearer and troubleshooting is faster. When a user can\u2019t send from Outlook, that is a different problem set from \u201cserver-to-server delivery is failing.\u201d Clean separation makes it easier to apply the right monitoring and limits to each path.<\/p>\n Both paths can harm deliverability if abused. A compromised mailbox using submission can generate high-volume spam, damaging IP and domain reputation. A misconfigured relay can be even worse, because it may allow outside actors to pump spam through your systems. Once reputation drops, legitimate business mail starts landing in spam folders or being rejected.<\/p>\n Submission logs usually map clearly to a user identity. Relay logs may not, unless you design for traceability\u2014such as using dedicated accounts for applications, segregating sending sources, or applying consistent tagging\/identification at the gateway level.<\/p>\n Use SMTP submission. Users move across networks, work remotely, and use many devices. Authentication and TLS are essential, and submission is built to support those requirements.<\/p>\n You typically have two good options:<\/p>\n The safest choice depends on your environment, but \u201clet anything on the network relay freely\u201d is not a safe default.<\/p>\n These devices sometimes have limited support for modern TLS\/authentication. In such cases, an internal relay can be appropriate\u2014but it should be tightly restricted to known device IPs and ideally limited in what it can send. Segmenting these devices away from general user networks reduces risk.<\/p>\n Public email delivery relies on relay behavior between MTAs. Here, DNS correctness and authentication (SPF\/DKIM\/DMARC) become essential, along with sensible queue management, bounce handling, and clear operational monitoring.<\/p>\n Open relay is one of the fastest ways to lose sending reputation. Secure relay design typically includes:<\/p>\n For submission, require TLS wherever possible. For relay\/transfer, don\u2019t assume TLS is always available unless you implement policies and monitoring that enforce or at least report encryption usage. Knowing where encryption is guaranteed vs opportunistic is important for risk management.<\/p>\n Account compromise, misconfigured apps, or runaway scripts can generate bursts of outbound mail. Rate limits and alert thresholds help protect deliverability and reduce operational incidents. Treat outbound volume as a signal: sudden spikes deserve investigation.<\/p>\n On submission, it\u2019s good practice to ensure users send only as permitted identities, or at minimum to detect and flag unexpected sender addresses. On relay, implement policies so devices and applications can\u2019t impersonate arbitrary internal addresses without approval. This supports brand trust and helps keep authentication alignment consistent.<\/p>\n When something goes wrong, you need to identify the source quickly. Helpful practices include:<\/p>\n A classic failure mode: a printer or app needs to send mail, so relay is enabled broadly and ends up reachable beyond the intended network boundaries. The outcome is often abuse, reputation damage, and legitimate mail being blocked.<\/p>\n Sometimes organizations try to make user clients send using transfer-style SMTP behavior. This creates headaches with authentication, roaming users, and security enforcement. Users should use submission; servers should do transfer\/relay.<\/p>\n SMTP submission is the authenticated, policy-controlled path for user sending. SMTP relay is the transport mechanism for server-to-server delivery and certain system integrations. Treating them as separate flows\u2014with the right authorization, TLS posture, monitoring, and rate limits\u2014reduces abuse risk and stabilizes deliverability.<\/p>\n If you\u2019re reviewing your organization\u2019s email setup on a platform like MX1Mail, start by inventorying every source that sends email (users, apps, devices) and explicitly deciding whether each one should use submission or relay. That one exercise often reveals hidden risks and leads to a cleaner, more secure outbound mail architecture.<\/p>\n","protected":false},"excerpt":{"rendered":" Learn the difference between SMTP relay and SMTP submission, when to use each in business environments, and practical steps to secure outbound email and protect deliverability.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[217],"tags":[281,114,226],"class_list":["post-136","post","type-post","status-publish","format-standard","hentry","category-email-infrastructure","tag-deliverability","tag-e-posta-guvenligi","tag-smtp"],"_links":{"self":[{"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/posts\/136","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/comments?post=136"}],"version-history":[{"count":0,"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/posts\/136\/revisions"}],"wp:attachment":[{"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/media?parent=136"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/categories?post=136"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mx1mail.com\/blog\/wp-json\/wp\/v2\/tags?post=136"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}\n
What is SMTP relay?<\/h2>\n
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Critical differences between relay and submission<\/h2>\n
1) Authentication and authorization approach<\/h3>\n
2) Separating traffic types for better control<\/h3>\n
3) Deliverability impact<\/h3>\n
4) Traceability and logging<\/h3>\n
Which one should you use? Practical scenarios<\/h2>\n
End users: Outlook, mobile, desktop clients<\/h3>\n
Websites and applications (contact forms, ERP\/CRM, monitoring tools)<\/h3>\n
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Printers, scanners, and legacy devices<\/h3>\n
Server-to-server transfer on the internet<\/h3>\n
A secure configuration checklist for businesses<\/h2>\n
Eliminate open relay risk<\/h3>\n
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Standardize TLS expectations<\/h3>\n
Apply rate limiting and anomaly detection<\/h3>\n
Control and validate \u201cFrom\u201d addresses<\/h3>\n
Make logs actionable<\/h3>\n
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Common mistakes and their real-world consequences<\/h2>\n
Opening relay \u201cjust for a device\u201d and leaving it exposed<\/h3>\n
Forcing users to send via relay-style transfer settings<\/h3>\n
Conclusion: choosing the right sending path improves security and deliverability<\/h2>\n