Handle SMTP 550 recipient-rejected errors with structured SmtpSendException

- Add SmtpSendException with smtpCode/smtpResponse/isRecipientRejected()
- smtpSend() $expect closure throws SmtpSendException (with code) instead of RuntimeException
- SmtpRelay::send() re-throws SmtpSendException so callers can inspect it
- request-access.php (new): catch 550 → roll back token+approval, return HTTP 422 with FR user message
- request-access.php (resend): catch 550 → HTTP 422 instead of silently claiming success
- StudentEmail::sendConfirmation(): catch SmtpSendException → log+false (submission not aborted)
- admin/actions/access-request.php: catch SmtpSendException post-approval → flash warning (recipient-rejected vs transient)

app/src/SmtpRelay.php

@@ -16,6 +16,31 @@ class SmtpProbeException extends \RuntimeException {
     }
 }
 
+/**
+ * Structured exception for SMTP send failures.
+ * Carries the 3-digit SMTP response code and the full server response.
+ *
+ * Notable codes:
+ *   550 / 551 / 553 — recipient address rejected (unknown user, policy, etc.)
+ *   421 / 450 / 451 — transient failures (try again later)
+ *   530 / 535       — authentication failure
+ */
+class SmtpSendException extends \RuntimeException {
+    public function __construct(
+        string $message,
+        public readonly int    $smtpCode   = 0,
+        public readonly string $smtpResponse = ''
+    ) {
+        parent::__construct($message);
+    }
+
+    /** True when the SMTP server permanently rejected the recipient address. */
+    public function isRecipientRejected(): bool
+    {
+        return in_array($this->smtpCode, [550, 551, 552, 553, 554], true);
+    }
+}
+
 /**
  * SMTP Relay — credentials stored in the DB, sending via a native PHP socket
  * client (no external dependencies).
@@ -391,6 +416,9 @@ class SmtpRelay {
 
         try {
             return self::smtpSend($s, $to, $rawMessage);
+        } catch (SmtpSendException $e) {
+            error_log('[SmtpRelay] ' . $e->getMessage());
+            throw $e;  // propagate structured exception so callers can react
         } catch (\Throwable $e) {
             error_log('[SmtpRelay] ' . $e->getMessage());
             return false;
@@ -463,7 +491,12 @@ class SmtpRelay {
         $expect = function (string $cmd, string $code) use ($send): string {
             $resp = $send($cmd);
             if (strncmp($resp, $code, strlen($code)) !== 0) {
-                throw new \RuntimeException("SMTP unexpected response to '{$cmd}': {$resp}");
+                $respCode = (int) substr(trim($resp), 0, 3);
+                throw new SmtpSendException(
+                    "SMTP unexpected response to '{$cmd}': " . trim($resp),
+                    $respCode,
+                    trim($resp)
+                );
             }
             return $resp;
         };

app/src/StudentEmail.php

@@ -99,7 +99,13 @@ class StudentEmail {
         $subject   = 'Merci — ton TFE a bien été enregistré';
         $htmlBody  = self::buildHtml($thesis);
 
-        $result = SmtpRelay::send($db, $to, $subject, $htmlBody);
+        try {
+            $result = SmtpRelay::send($db, $to, $subject, $htmlBody);
+        } catch (SmtpSendException $e) {
+            // Confirmation email failure must not abort the successful submission.
+            error_log("[StudentEmail] SMTP error sending to {$to} for thesis #{$thesisId}: " . $e->getMessage());
+            return false;
+        }
 
         if ($result) {
             error_log("[StudentEmail] Confirmation sent to {$to} for thesis #{$thesisId}");