SMTP enhanced status code 4.2.4

What the registry says

The basic codes it travels with

• 450 Requested mail action not taken: mailbox unavailable (e.g., mailbox busy or temporarily blocked for policy reasons)
• 452 Requested action not taken: insufficient system storage

In practice

X.2.4 appears when you send to a list address, such as a group or a distribution list, and the list server cannot expand it into its members, often because of a loop, a missing member database or a configuration error. It is not about you or your list. If it persists, the list owner needs to fix the list, and a direct message to the owner is usually the fastest route.

What the sender should do

Retry later for a 4.2.4. For a 5.2.4, contact the list owner; the problem is on their side.

Common questions

What does 4.2.4 mean?

X.2.4 means the recipient is a mailing list address and the list could not be expanded to its members. It can be temporary or permanent, depending on why the list failed.

What should I do about 4.2.4?

Retry later for a 4.2.4. For a 5.2.4, contact the list owner; the problem is on their side.

Would an email verifier have caught 4.2.4?

No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.