SMTP relay for applications
Transactional SMTP relay built for important app email
Deliver receipts, account alerts, password resets and other one-to-one messages through authenticated SMTP, with domain authentication and delivery logs to help you operate confidently.
Two-day free trial on eligible plans. Connect with standard SMTP credentials—no proprietary sending API required.
A practical SMTP service for transactional email
Keep application sending straightforward while gaining the controls needed to diagnose delivery and protect your domain.
Works with standard SMTP
Use the SMTP library already supported by your application, framework, ecommerce platform or monitoring tool.
Authenticate your sending domain
Configure the DNS records shown in your account to support SPF, DKIM and DMARC alignment for your domain.
Review delivery activity
Use message logs and SMTP responses to investigate rejected, deferred or successfully accepted messages.
Send and receive with one service
Pair application sending with hosted mailboxes so customers can reply to a real address on your domain.
Reliable application delivery
What is a transactional SMTP relay?
A transactional SMTP relay accepts messages from your application after authentication, then handles their onward delivery to recipient mail servers. It is designed for messages triggered by a person or system event, such as an order confirmation, security notice, invoice or sign-in link.
SMTP Zen gives your application a conventional SMTP endpoint rather than locking it to a proprietary API. Use encrypted connections on port 465 with implicit TLS or port 587 with STARTTLS. Transactional delivery still depends on responsible sending, accurate recipient data and sound message content; an SMTP relay cannot guarantee inbox placement.
Connect your application in three steps
The same basic setup works across common languages, frameworks and hosted applications.
- 1
Add and verify your domain
Add the domain in SMTP Zen, then publish the supplied DNS records and wait for verification.
- 2
Create SMTP credentials
Use a mailbox identity and copy its hostname, username, password, port and encryption settings into your application.
- 3
Send a test and inspect the logs
Trigger a representative message, confirm the From address and reply path, then check the recorded delivery response.
Designed for operational email, not indiscriminate outreach
Good transactional delivery begins with a clear message purpose and a recipient who expects to receive it.
Appropriate sending patterns
Account notifications, booking updates, invoices, form confirmations and monitoring alerts are natural uses. Purchased lists, unsolicited cold email and high-volume marketing campaigns are not transactional traffic.
Authentication and reputation
SPF and DKIM help receiving systems verify the sending source, while DMARC defines how authentication failures should be handled. These controls support trust, but list quality, complaint rates and content also affect delivery.
Replies and support workflows
A hosted mailbox gives the From or Reply-To address somewhere to receive customer responses. Configure forwarding aliases when several public addresses should reach the same team inbox.
Who transactional SMTP relay is for
SaaS and web applications
Send sign-in links, billing notices, invitations and product notifications from a domain your users recognise.
Ecommerce platforms
Deliver order confirmations, dispatch updates, receipts and customer-service replies without relying on a web server’s local mail function.
Developers and small teams
Use familiar SMTP configuration and readable logs without building a mail-transfer stack yourself.
Transactional SMTP relay questions
Put your application email on a proper SMTP relay
Start with your own domain, test a real transactional workflow and review the delivery activity before switching production traffic.
Start sending transactional email