I have this in docker-compose.yml:

  smtp:
    image: namshi/smtp
    container_name: ...smtp_relay_1
    restart: "no"
    ports:
      - "25:25"

In the docker-compose output I get no error.

I get this error in the browser using the JavaScript code below:

There was an error trying to send your message. Please try again later.

The JavaScript with the custom usage of the Contact Form 7 REST API is this:

prepareContactForm() {
    $(".wpcf7-submit").on("click", (e) => {
        e.preventDefault();
        $.post("/wp-json/contact-form-7/v1/contact-forms/51/feedback",
                {
                    "your-name": "Somebody",
                    "your-email": "...my-email-address-here...",
                    "your-subject": "tEsTiNg",
                    "your-message": "a\nb\nc"
                }).then(
            (data) => {
                let $el = $(".wpcf7-response-output");

                if (data.status === "validation_failed") {
                    $el.text(data.message);
                    $el.removeClass("wpcf7-display-none");
                    return;
                }

                if (data.status === "mail_failed") {
                    $el.text(data.message);
                    $el.removeClass("wpcf7-display-none");
                    return;
                }

                if (typeof data.invalidFields !== "undefined") {
                    let output = "";
                    for (let i = 0; i < data.invalidFields.length; ++i) {
                        let into = data.invalidFields[i].into;
                        let message = data.invalidFields[i].message;

                        output += <code>${into} :: ${message}\n</code>;
                    }

                    if (output.length > 0) {
                        $el.text(output);
                        $el.removeClass("wpcf7-display-none");
                        return;
                    }
                }

                console.log('log', data);

                $el.text("The message has been sent.");
                $el.removeClass("wpcf7-display-none");
            }
        );
        return false;
    });
}

The form #51 is configured to send emails to my email address… But I checked the Spam folder too and I get no email from this form. Is there any configuration I need to do for SMTP inside the WordPress admin interface?

Thank you.