It's a one line patch so should be safe enough to add manually if anyone wants to try it:
diff --git a/net-mgmt/pfSense-pkg-arpwatch/files/usr/local/pkg/arpwatch.inc b/net-mgmt/pfSense-pkg-arpwatch/files/usr/local/pkg/arpwatch.inc
index 247cc3c4e1f4baf6325c22ab778d64c3bdf8afc2..e4c2f2b9e72d96a573c7ebb3ce52c01c278265ab 100644
--- a/net-mgmt/pfSense-pkg-arpwatch/files/usr/local/pkg/arpwatch.inc
+++ b/net-mgmt/pfSense-pkg-arpwatch/files/usr/local/pkg/arpwatch.inc
@@ -239,7 +239,7 @@ if ((false !== $message) && ((false === strpos($message, ': Cron ')) ||
$message = preg_replace('/^To: .*$/m', '', $message);
$message = preg_replace('/^Subject: .*$/m', '', $message);
$message = preg_replace("/^(\n){4}/", '', $message);
- $send_subject = config_get_path('system/hostname') . config_get_path('system/domain') . " - Arpwatch Notification : {$subject[1]}";
+ $send_subject = config_get_path('system/hostname') . "." . config_get_path('system/domain') . " - Arpwatch Notification : {$subject[1]}";
send_smtp_message($message, $send_subject);
if (function_exists('notify_via_telegram')) {
https://redmine.pfsense.org/issues/16410