What does the "Inbox Full" result status mean?

Inbox Full is the status the Valid Email Checker engine returns when a mailbox is over its storage quota at the moment of verification. The address exists, the owner is presumably still using it, but the mail server is rejecting incoming messages because there is literally no room to store them. Step 10 of the 11-step engine catches this.

In the canonical enum, this appears as inbox_full alongside safe, invalid, unknown, catch_all, disposable, role, spamtrap, and disabled. It is one of the recoverable statuses — unlike Invalid or Disabled, the underlying mailbox might be fully functional again next week.

How the engine detects it

Mail servers respond to a probe with specific error codes when the mailbox is over quota. The most common is 552 ("Requested mail action aborted: exceeded storage allocation"). Some providers use 422 or 452 instead. Reading the response code and matching it against the standard quota-related errors is what triggers the Inbox Full classification.

A few providers do not signal quota cleanly and instead respond with a generic 4xx or 5xx error. When the response is ambiguous, the engine falls back to Unknown and refunds the credit, rather than guess at the underlying cause.

What Inbox Full tells you about the recipient

• The recipient still has an account but is not paying attention to housekeeping. Active power-users rarely let mailboxes fill to the cap.
• On free consumer plans (older Hotmail, free Yahoo) the storage cap is small enough that hitting it is common. Business mailboxes hit it less often.
• The address may bounce in and out of Inbox Full as the recipient deletes and accumulates mail.
• Engagement on this address is usually weak. A mailbox at quota is rarely getting read regularly.

How to handle Inbox Full in your campaign

The safe move is to skip Inbox Full entries on this send and re-verify in one to four weeks. If the mailbox cleared out, the next pass will return Safe and you can mail normally. If it stayed full or changed to Disabled, the engine catches that too.

Some teams collapse Inbox Full into a low-priority "send last" segment and mail it once a quarter. That is reasonable for newsletters — the marginal cost is small and the small fraction that re-engages is a win. For cold outreach, Inbox Full is functionally a soft Invalid, because even if the message lands, the recipient is probably not reading.

What this means for your bounce rate

Sending to a known Inbox Full address often produces a soft bounce immediately. Most ESPs handle soft bounces gracefully on the first try and only count them against your reputation after repeated failures. But repeated sending to the same over-quota address eventually triggers reputation damage, so the cleanest practice is to suppress and re-test rather than retry blindly. See how to handle soft bounces vs hard bounces for the broader retry strategy and result types explained for the full status matrix.