Set webhook email sender identity

index.php

@@ -319,12 +319,13 @@ HTML;
 $emailHeaders = "MIME-Version: 1.0\r\n";
 // Content-type specifies HTML email with UTF-8 character encoding
 $emailHeaders .= "Content-type: text/html; charset=UTF-8\r\n";
-// From header sets the sender address to a no-reply address at wh.dixon.cx
-$emailHeaders .= "From: no-reply@wh.dixon.cx\r\n";
-
-// Send the email using PHP's built-in mail function
-// Parameters: recipient, subject, message body, headers
-$mailSent = mail($email, $subject, $html, $emailHeaders);
+// Keep both the visible sender and SMTP envelope sender on the public domain.
+$envelopeSender = 'no-reply@dixon.cx';
+$emailHeaders .= "From: Webhook Call <$envelopeSender>\r\n";
+
+// Send the email using PHP's built-in mail function
+// The -f option prevents sendmail from deriving www-data@<container-hostname>.
+$mailSent = mail($email, $subject, $html, $emailHeaders, '-f' . $envelopeSender);
 
 // Check if the email was sent successfully
 if (!$mailSent) {

tests/test_mail_sender.sh

@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+project_dir=$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd -P)
+test_dir=$(mktemp -d)
+trap 'rm -rf "$test_dir"' EXIT
+
+sendmail_args="$test_dir/sendmail.args"
+sendmail_message="$test_dir/sendmail.message"
+fake_sendmail="$test_dir/sendmail"
+prepend_file="$test_dir/prepend.php"
+
+cat > "$fake_sendmail" <<'FAKE_SENDMAIL'
+#!/usr/bin/env bash
+printf '%s\n' "$@" > "$MAIL_TEST_ARGS"
+cat > "$MAIL_TEST_MESSAGE"
+FAKE_SENDMAIL
+chmod +x "$fake_sendmail"
+
+cat > "$prepend_file" <<'PHP'
+<?php
+if (!function_exists('getallheaders')) {
+    function getallheaders(): array
+    {
+        return [];
+    }
+}
+PHP
+
+MAIL_TEST_ARGS="$sendmail_args" \
+MAIL_TEST_MESSAGE="$sendmail_message" \
+REQUEST_URI='/recipient%40example.com' \
+REQUEST_METHOD='POST' \
+REMOTE_ADDR='192.0.2.1' \
+CONTENT_TYPE='application/json' \
+php \
+    -d "auto_prepend_file=$prepend_file" \
+    -d "sendmail_path=$fake_sendmail -t -i" \
+    "$project_dir/index.php" >/dev/null
+
+if ! grep -Fxq -- '-fno-reply@dixon.cx' "$sendmail_args"; then
+    echo 'Expected the envelope sender to be no-reply@dixon.cx.' >&2
+    echo 'Actual sendmail arguments:' >&2
+    sed 's/^/  /' "$sendmail_args" >&2
+    exit 1
+fi
+
+if ! grep -Fq $'From: Webhook Call <no-reply@dixon.cx>\r' "$sendmail_message"; then
+    echo 'Expected the From header to be Webhook Call <no-reply@dixon.cx>.' >&2
+    exit 1
+fi
+
+echo 'Mail sender test passed.'