A recipient whose mailbox is full may still be a valid and willing recipient. The immediate task is to determine whether the original message remains queued, whether the receiving system considers the failure temporary, and whether the message will still be useful when space becomes available. Adding an SPF include will not create mailbox capacity.
Read the class and the capacity diagnostic
RFC 3463 defines X.2.2 as mailbox full and says it should be used as a persistent transient failure. The first component supplies the success, persistent transient failure or permanent failure class. Preserve the complete SMTP reply as well as the enhanced status: a dashboard's “soft bounce” label may hide the actual response and transaction stage. RFC 3463 section 3.3.
A 4xx reply normally leaves responsibility for retry with the sending system. A 5xx reply indicates permanent failure for that attempt and should not trigger routine repeated attempts without intervention. If the three-digit SMTP class and enhanced class disagree, retain both and investigate the server or parser behavior rather than silently rewriting the evidence. The underlying storage condition can later change even when a particular attempt has permanently failed.
Establish which message remains queued
Capture the sender's queue identifier, original submission time, failed envelope recipient, last response and next scheduled attempt. A multi-recipient message can have one deferred recipient while others have already been accepted. Resending to the complete visible recipient list can therefore duplicate messages for people unaffected by the capacity problem.
Check whether the sender still owns the delivery attempt or whether a downstream relay accepted responsibility and later generated a notification. A delay warning is not necessarily a final failure. Use the queue record and recipient-specific notification fields together. Keep original submission time separate from notification arrival time, especially when the sender reports that the failure happened “just now.”
Work through a hypothetical delayed message
Suppose queue item Q378 receives 452 4.2.2 for one recipient. The recipient's administrator later confirms an approved capacity repair. Before creating a replacement, check Q378: if it still awaits a scheduled retry, follow that queue; if its later attempt has already received final DATA acceptance, an old warning is not a reason to send another copy.
If Q378 expired before capacity was restored, retain its failed outcome and identify any authorized replacement separately. The repair does not retroactively deliver the expired item. Other recipients may already have succeeded, so avoid resending the entire visible recipient list. These cases are hypothetical, not a live mailbox test, retained queue or measured recovery.
Resolve the capacity issue without multiplying mail
For an organization-controlled recipient, the mailbox owner or administrator should review quota, retention and available storage through approved procedures. Do not delete user data merely to clear a delivery alert. For an external recipient, use an established alternative channel when the business message is urgent, and let that person resolve their own mailbox conditions.
After remediation, first check whether the original queued item has already succeeded. Initiate a new authorized submission only when its relationship to the original attempt is understood. Respect the sender's documented retry and expiry policy rather than hammering the receiver. RFC 5321 describes retry intervals and configurable give-up behavior; its general guidance is not a promise that every provider retains mail for the same period. RFC 5321 section 4.5.4.1.
Close the incident with the recipient disposition, capacity correction and final delivery evidence. SMTP acceptance establishes a handoff, not proof that the recipient read the message. If the original has expired, record that fact separately from any successfully submitted replacement.
Decide whether a replacement is needed
- Original still queued: check next retry and usefulness before making another copy.
- Original accepted after repair: do not resend merely because an old warning arrived.
- Original expired: record that outcome separately; authorize a replacement only if still useful.
- Mixed recipients: limit any new send to the intended unresolved recipients.
- Capacity repair: retain owner-approved evidence without exposing mailbox contents.