GuideSMTP Relay

SMTP Relay Configuration

Learn how to obtain your SMTP credentials and configure your existing applications or servers to send transactional email through Simply Send.

Transactional emails only. SMTP relay is supported exclusively for transactional emails — system-triggered messages such as password resets, order confirmations, and account notifications. For marketing emails and campaigns, use the Marketing Email API instead.

Availability restrictions: SMTP relay access is enabled once your sender reputation is established. Verify your sending domain, complete your account setup, and send your first emails via the API to build a sending history before SMTP credentials become active.

SMTP email size: The standard limit is 2 MB per email. With an approved workspace limit, SMTP supports a final email of up to 40 MB (40,000,000 bytes), including the body, headers, MIME overhead, and Base64-encoded attachments. The body and other non-attachment data must stay within 2 MB. You can attach up to 10 files; the attachments alone cannot total 40 MB.

For larger emails, go to Account → Approvals, select Transactional Email Size Increase, and describe the total email size and expected sending volume you need. Wait for approval and the workspace limit update before sending. Use the same SMTP host and ports for both standard and approved larger emails.

1. Get Your SMTP Credentials

To start sending via SMTP, you'll need your unique SMTP credentials.

Log in to your Simply Send Dashboard.
Navigate to the SMTP on the left menu.
Create a new SMTP credentials. Keep this credentials secure.

2. Connection Settings

Use these settings in your email client, server configuration, or application code.

SMTP Host

smtp.simplysend.email

Port (STARTTLS)

587

Port (SSL/TLS)

465

Authentication

Required

3. Implementation Examples

Important note about BCC: Email libraries like Swaks and Nodemailer require a To field for the MIME message structure. When you provide a Bcc field, Simply Send will ignore the To field and send the email as BCC-only. The same applies to Cc — if both Cc and Bcc are present, only Bcc will be used.

Idempotency (optional)

Add an Idempotency-Key MIME header with a unique UUID to prevent duplicate sends when your client retries. The header is stripped before delivery and is not forwarded to recipients.

Use a new UUID for each distinct email. Reuse the same key only when retrying the same send.
A retry with the same key within 24 hours returns the original result and does not send a second email.
If the original send is still in progress, the SMTP server returns a 450 temporary failure. Retry later with the same key.

Swaks (Bash)

Standard Email Send (To recipient visible to all)

BCC-only Send (Recipient hidden from other recipients)

Swaks requires the --to flag for the envelope recipient. When you also specify --bcc, Simply Send will use BCC for delivery and ignore the To field.

Python (smtplib)

Standard Email Send (To recipient visible to all)

BCC-only Send (Recipient hidden from other recipients, no To header in MIME)

Python's email library allows creating messages without a To header. You can use sendmail() with explicit envelope recipients for true BCC-only sends.

Node.js (Nodemailer)

Standard Email Send (To recipient visible to all)

BCC-only Send (Recipient hidden from other recipients)

Nodemailer requires a to field in the options object. When you also provide bcc, Simply Send will use BCC for delivery and ignore the To field.

Java (Jakarta Mail)

Requires the jakarta.mail dependency (for example com.sun.mail:jakarta.mail). Attachments must use a single To recipient — do not combine attachments with Cc or Bcc. SMTP works the same way for all supported message sizes. See the email-size and approval requirements at the top of this guide before sending more than 2 MB.

Send with attachment

C# (MailKit)

Install MailKit with dotnet add package MailKit. Set the SIMPLYSEND_ACCOUNT_ID and SIMPLYSEND_SMTP_PASSWORD environment variables to your SMTP credentials. This example uses STARTTLS on port 587.

Standard Email Send (To recipient visible to all)

C# (System.Net.Mail)

This compatibility example uses .NET's built-in System.Net.Mail APIs and STARTTLS on port 587. Microsoft does not recommend SmtpClient for new development; use the MailKit example above for new integrations.

Set SIMPLYSEND_ACCOUNT_ID (the SMTP username) and SIMPLYSEND_SMTP_PASSWORD as environment variables; load the password from a secret manager in production. Replace the sender and recipient placeholders, and use a sender address on a verified domain.

SMTP errors propagate from SendMailAsync. In a hosted app, handle and log failures with your standard logger without logging credentials or message content; follow the idempotency guidance above when adding retries.

Standard Email Send (To recipient visible to all)

4. Supported Headers

The SMTP relay extracts the following headers from your MIME message and forwards them to the API:

To — recipient email address(es)
Cc — CC recipient email address(es)
Bcc — BCC recipient email address(es). When Bcc is present, To and Cc must be omitted.
From — sender email address (must be a verified domain). Supports display name format: "Display Name" <[email protected]>
Reply-To — reply-to email address
Subject — email subject line
Idempotency-Key — optional unique UUID. Retries with the same key within 24 hours return the original result without sending again. The header is not forwarded to recipients.
Attachments — MIME attachments are extracted and forwarded
Custom headers — any other headers (except standard ones like Content-Type, DKIM-Signature, and Idempotency-Key)

BCC-only constraint: When sending with a Bcc header, the To and Cc headers must not be present. Use either To/Cc or Bcc, not both.

5. Troubleshooting

TLS / Root certificate errors

The SMTP endpoint smtp.simplysend.email uses a TLS certificate issued by Let's Encrypt. If your email client or platform reports that the root certificate cannot be found, it means the system does not yet trust the ISRG Root X1 certificate authority.

Download the ISRG Root X1 certificate from https://letsencrypt.org/certs/isrgrootx1.pem.
Install it into your system's trusted root certificate store (or ask your IT/security team to do so).
If your environment uses a TLS-inspecting proxy or strict outbound firewall, whitelist smtp.simplysend.email and allow outbound traffic on ports 587 and 465.

You can verify the certificate chain from a terminal:

A successful verification shows Verify return code: 0 (ok).

Authentication failures

Ensure you are using the account ID as the username and the SMTP password as the password from the Simply Send Dashboard.
Confirm that your sending domain is verified and that SMTP relay access has been enabled for your account.
Use PLAIN or LOGIN authentication. CRAM-MD5 is not supported.

Connection timeouts

Verify outbound access to smtp.simplysend.email on ports 587 and 465.
Ensure TLS 1.2 or higher is enabled. Older TLS versions are not supported.

Security

We support both STARTTLS on port 587 and Implicit TLS on port 465. TLS 1.2 or higher is required for all connections.

Compliance

Ensure your emails include an unsubscribe link and physical address.