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    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.

    1. Get Your SMTP Credentials

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

    1. Log in to your Simply Send Dashboard.
    2. Navigate to the SMTP on the left menu.
    3. 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

    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.

    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
    • Attachments — MIME attachments are extracted and forwarded
    • Custom headers — any other headers (except standard ones like Content-Type, DKIM-Signature, etc.)

    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.

    1. Download the ISRG Root X1 certificate from https://letsencrypt.org/certs/isrgrootx1.pem.
    2. Install it into your system's trusted root certificate store (or ask your IT/security team to do so).
    3. 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