diff --git a/src/mail.js b/src/mail.js
index 365c04003..ed877748c 100644
--- a/src/mail.js
+++ b/src/mail.js
@@ -397,6 +397,7 @@ async function checkDmarc(domain) {
     return dmarc;
 }
 
+// TODO: check ip6.arpa for IPv6 PTR
 async function checkPtr(mailFqdn) {
     assert.strictEqual(typeof mailFqdn, 'string');