Reply-To and submission results

Mail accepts an optional final replyTo: Sender. An explicit reply address reaches Postmark, Mailgun and SMTP. Without one, SMTP and Postmark retain their sender fallback; Mailgun omits the explicit Reply-To header.

$result = $mailer->sendWithResult(
    $mail,
    metadata: ['job_id' => '123'],
    trackOpens: true
);

SubmissionMailerInterface adds this optional capability without changing the existing MailerInterface::send(): bool contract. MailSubmission contains messageId, submittedAt and accepted. A message identifier or provider timestamp can be null when the transport does not supply it.

Acceptance means the transport accepted submission, not that a recipient received the message. Provider rejection remains an exception where the SDK reports an error. SMTP can return accepted: false on transport refusal. Existing boolean sends delegate to submission results and preserve their transport behavior.

Postmark and Mailgun pass metadata and open-tracking settings to their providers. SMTP cannot provide those provider features and ignores the optional settings. Correlate SMTP submissions with their message identifier instead. Tests inspect SDK and SMTP payloads without sending actual mail.