Yahoo [GL01]: message from IP temporarily deferred

What it means

GL01 is a deferral that usually reads like the example. Yahoo does not list GL01 by name in its published SMTP error reference, which describes 421 deferrals in general as caused by unusual traffic patterns, complaints, spam characteristics, or busy servers. Senders who report GL01 see it when Yahoo meets traffic from an IP that does not fit what it expects, often an IP with little or no history at Yahoo, or one whose volume changed sharply. Because the reply is 421, your server will retry.

GL01 is milder than TSS04 and much milder than TSS09. It tends to appear during IP warm-up or right after a migration to a new mail server or ESP, and fades as Yahoo learns that your mail is wanted. Persistent GL01 responses can mean the mail is not getting the engagement Yahoo looks for.

Likely causes

• A new sending IP, or one that has been idle for a long time, starting to send to Yahoo.
• A sudden change in volume, such as a monthly newsletter going out all at once.
• Moving to a new ESP or server without gradually ramping traffic to Yahoo addresses.
• Low engagement from Yahoo recipients for recent campaigns.

Checks that apply

• Reverse DNS (PTR) Ensures the new IP has forward-confirmed reverse DNS, which helps Yahoo trust unfamiliar traffic.
• Google/Yahoo Compliance Verifies the shared Yahoo and Gmail sender requirements before you scale up.
• MailFlow Monitor Lets you watch Yahoo acceptance of test mail as you warm up.

FAQ

Is GL01 a blacklist?

No. It is a temporary deferral based on traffic patterns. Messages are usually accepted on a later retry.

How is GL01 different from TSS04?

GL01 points to unfamiliar or unusual traffic. TSS04 points to volume combined with complaints, a stronger reputation signal.