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Sendmux accepts email via standard SMTP, making it compatible with any language, framework, or email library that supports SMTP delivery.

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Connection details

SettingValue
Hostsmtp.sendmux.ai
Port587 (STARTTLS) or 465 (implicit TLS)
EncryptionTLS required
Usernamesmtpuser (or any value — username is ignored)
PasswordYour API key (smx_...)
AuthenticationLOGIN or PLAIN

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Examples

import smtplib
from email.mime.text import MIMEText

msg = MIMEText("<h1>Hello!</h1>", "html")
msg["Subject"] = "Test email"
msg["From"] = "hello@yourdomain.com"
msg["To"] = "user@example.com"

with smtplib.SMTP("smtp.sendmux.ai", 587) as server:
    server.starttls()
    server.login("smtpuser", "smx_your_key_here")
    server.send_message(msg)

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When to use SMTP vs HTTP API

SMTPHTTP API
Best forLegacy systems, existing email librariesNew integrations, batch sending
Batch sendingOne connection per emailUp to 100 per request
AttachmentsNative MIMEBase64-encoded JSON
IdempotencyNot supportedBuilt-in idempotency keys
Response formatSMTP status codesStructured JSON
For new integrations, the HTTP API is recommended. Use SMTP when integrating with systems that only support SMTP delivery (e.g. WordPress, legacy CRM systems).