How to handle Gmail mailbox-full bounces
Separate a recipient storage problem from an invalid address, and handle retries and suppression using the full SMTP response.
A storage-related bounce tells you why a particular delivery failed. Before deciding what to do with the contact, keep the full SMTP response and check how your sending platform classified it.
Why a small inbox can still be out of space
A personal Google Account shares storage across Gmail, Google Drive and Google Photos. Reaching the account's storage limit can affect Gmail's ability to receive mail, even when the recipient believes the inbox itself contains little. Source: How Google storage works.
Google's SMTP reference includes 552 5.2.2 for an inbox that is out of storage and inactive. Read the response you received rather than assuming every provider's “mailbox full” label has the same meaning. Source: Gmail SMTP errors and codes.
Separate delivery handling from contact status
SMTP distinguishes temporary 4xx failures from permanent 5xx failures. A temporary response allows a later retry; a permanent response says the request failed and should not simply be repeated unchanged. A condition can sometimes be corrected later without changing the meaning of the original failure. Source: SMTP reply semantics, RFC 5321.
Keep two fields in your own investigation: the delivery outcome and the reason. “Failed: recipient storage” is more useful than merging all failures into “invalid email.” Preserve any additional information in the reply, including an inactivity indication.
Use the sending platform's controls
For temporary failures, let the platform manage its documented queue and retry schedule. Check whether new campaigns continue adding messages for the same failing recipient.
For a permanent failure, review the platform's suppression decision. Do not export the contact into another campaign or provider to get around that suppression. If the recipient later confirms the problem is fixed and requests mail again, use the provider's supported review process.
For marketing, my recommendation is to exclude repeatedly failing contacts from routine campaigns while you review their status. Record the first and latest failures and the number of affected sends. Set your review policy with the ESP; an arbitrary seven-day pause is not a universal Gmail rule.
Handle essential messages separately
If an account message fails, offer an existing in-app route or another contact method the customer has provided for that purpose. Sending another notice to the same full mailbox may leave the customer without the information they need.
When an individual reports that receiving mail works again, verify the address and the request before arranging a fresh message. Keep unsubscribe choices intact throughout the process.
Related guides
Gmail SMTP error codes: what they mean and how to fix them
- How to investigate Gmail delivery errors
- How to manage subscriptions without creating Gmail complaints