diff --git a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
--- a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
+++ b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
@@ -1516,6 +1516,37 @@
         $new_texts);
 
       $phids = array_diff($new_phids, $old_phids);
+
+      // Count any users who were explicitly added as subscribers (other than
+      // the actor) as "mentioned". If someone: just adds another user as a
+      // subscriber; or adds them as a subscriber explicitly, then writes a
+      // comment without actually "@mentioning" them, we still want it to
+      // behave like a mention for the purposes of mail stamps.
+
+      $subscriber_type = PhabricatorTransactions::TYPE_SUBSCRIBERS;
+      $old_state = array_values($this->subscribers);
+      $old_state = array_fuse($old_state);
+      foreach ($xactions as $xaction) {
+        if ($xaction->getTransactionType() != $subscriber_type) {
+          continue;
+        }
+
+        $new_value = $xaction->getNewValue();
+
+        $new_phids = array_merge(
+          idx($new_value, '=', array()),
+          idx($new_value, '+', array()));
+
+        foreach ($new_phids as $phid) {
+          if ($phid === $this->getActingAsPHID()) {
+            continue;
+          }
+
+          if (!isset($old_state[$phid])) {
+            $phids[] = $phid;
+          }
+        }
+      }
     } else {
       $phids = array();
     }