SMTP submissions for transactional email

In addition to the HTTP API, Flexmail supports sending transactional email via SMTP. This is useful when your application or platform already has SMTP support built in — a CMS, an e-commerce platform, or a legacy system that sends email through an SMTP relay.

SMTP submission is included in the transactional product at no extra cost. It provides the same delivery reliability and tracking capabilities as the API, with a few differences in defaults and supported features.

Note

SMTP submission is not enabled by default. You need to request access explicitly. See "Requesting access" below.

Requesting access

SMTP access is not enabled automatically when you activate the transactional product. To request SMTP credentials for your account, contact Flexmail support at support@flexmail.eu and include your account ID in the request. Your SMTP username and password will be provided as part of the credentials response.

Note

Your SMTP credentials are separate from your personal access token. The username and password for SMTP are provided by Flexmail upon request — do not use your account ID and personal access token as SMTP credentials.

Connection settings

Once your SMTP access has been granted, use the following settings:

Setting	Value
Server	submission.flexmail.eu
Port	587
Connection security	STARTTLS
Username	Provided by Flexmail on request
Password	Provided by Flexmail on request

Custom headers

SMTP submissions support several optional custom headers that control message behaviour and tracking. These work similarly to the options available in the HTTP API.

Header	Description
X-Flexmail-Tag	Add a tag to categorise messages and accumulate stats. Must be in snake_case format (1–50 characters, e.g. order_confirmation).
X-Flexmail-Metadata-{key}	Add custom metadata as key/value pairs. Replace {key} with your field name in snake_case. Value must be 1–80 characters.
X-Flexmail-Open-Tracking	Set to true to enable open tracking. Disabled by default.
X-Flexmail-Link-Tracking	Set to true to enable link tracking. Disabled by default.

Example:

X-Flexmail-Tag: order_confirmation
X-Flexmail-Metadata-order_id: 12345
X-Flexmail-Metadata-customer_id: C-789
X-Flexmail-Open-Tracking: true
X-Flexmail-Link-Tracking: true

Note

Open and link tracking are disabled by default for both SMTP submissions and the HTTP API. Add the corresponding headers if you want tracking enabled.

Message ID and webhook correlation

The original Message-ID header from your submitted email is automatically stored as metadata under the key original_message_id. This lets you correlate webhook delivery events with the messages you sent from your own system.

SMTP vs. the HTTP API

The two methods have some differences in defaults and capabilities:

Feature	HTTP API	SMTP submission
Open tracking	Disabled by default	Disabled by default
Link tracking	Disabled by default	Disabled by default
Original Message-ID	N/A	Preserved as original_message_id metadata
Inline attachments	Supported	Not supported
Plain text only email	Supported	Not supported

Supported features

HTML emails with or without a plain text multipart.
Open and link tracking (via custom headers).
CC and BCC recipients.
Attachments — must be base64-encoded. Inline attachments are not supported.
Rate limiting per recipient, consistent with the API rate limits.

Requirements

The From address must be a verified sender in your Flexmail account.
The sending domain must be authenticated with SPF, DKIM, and DMARC, and a return-path must be configured. These requirements are the same as for the HTTP API.

Error handling

Submission errors are reported via an NDR (Non-Delivery Report) sent to the envelope sender address. Bounce events and all other message events are also available through the message details endpoint and webhooks, in the same way as API sends.

Next steps

See "Getting started with the transactional API" for the full account setup process.
See "Transactional webhooks" to receive real-time delivery and tracking events.
See "Transactional troubleshooting" if your SMTP connection or deliverability is not working as expected.