Index: src/wp-admin/includes/privacy-tools.php
===================================================================
--- src/wp-admin/includes/privacy-tools.php	(revision 52698)
+++ src/wp-admin/includes/privacy-tools.php	(working copy)
@@ -138,8 +138,18 @@
 					} else {
 						$email_address = $user->user_email;
 					}
-				} else {
-					$email_address = $username_or_email_address;
+				}  elseif ( is_email( $username_or_email_address ) ) {
+					$user = get_user_by( 'user_email', $username_or_email_address );
+					if ( ! $user instanceof WP_User ) {
+						add_settings_error(
+							'username_or_email_for_privacy_request',
+							'username_or_email_for_privacy_request',
+							__( 'Unable to add this request. A valid email address or username must be supplied.' ),
+							'error'
+						);
+					} else {
+						$email_address = $user->user_email;
+					}
 				}
 
 				if ( empty( $email_address ) ) {