When an email that your forum sends to a member is rejected by the member's email provider, we record the provider's response. If the failure is permanent, the member's email address is marked as invalid so the forum stops sending to it, protecting your forum's email reputation. This article lists the error responses we see most often, what each one means, and how to resolve it.
Where to see the error response
Forum administrators: open the Users page, click the email icon, select the user groups, and click the ignored email addresses link in the information box. Each address in the list is shown with the most recent response we received from the provider and the date it arrived.
You can also see every delivery failure for your forum, with the reason and the recommended action, on the email history page. The email history also covers queued, sent, and skipped emails, so it is the fastest way to confirm what happened to a specific email.
Members: when a member with a flagged address visits the forum, they are taken to their Preferences page and shown the response and date, along with a link to change their email address or re-enable email delivery.
Common error responses
The mailbox does not exist
Provider responses: "The email account that you tried to reach does not exist", "Invalid Recipient", "550 5.1.1 Recipient not found", "Requested action not taken: mailbox unavailable".
What it means: there is no mailbox at that address. Either the member typed the address incorrectly when signing up, or the account has since been closed or deleted.
How to resolve: the member should confirm the spelling of the address and, if it is correct, update their account with a different address on the Preferences page.
The mailbox has been disabled or suspended
Provider responses: "554 5.2.1 mailbox disabled", "Sorry, your message to this address cannot be delivered", "552 Requested mail action aborted, mailbox not found", "account has been suspended".
What it means: the mailbox exists but the provider has deactivated it, for example because the member's account is suspended, closed, or frozen.
How to resolve: the member should contact their email provider, or update their account with a working address.
The mailbox is full
Provider responses: "554 5.2.2 mailbox full", "QuotaExceededException", "The recipient's inbox is out of storage space", "user is over quota".
What it means: the mailbox has run out of storage, so the provider is rejecting new mail. This usually recovers once the member frees up space.
How to resolve: the member should delete old emails or empty their trash to free up space, then click re-enable email delivery in the message on their Preferences page. If the mailbox still cannot receive our email afterwards, the address will be marked as invalid again.
The organization blocks external forwarding
Provider response: "550 5.7.520 Your organization does not allow external forwarding. Please contact your administrator for further assistance."
What it means: the member's mailbox is set to auto-forward to an external address, and their organization (for example Microsoft 365 or Google Workspace) does not allow that. The forwarding rule rejects our email, so it never reaches them.
How to resolve: the member should remove the auto-forward rule and read their mail directly at this address, or update their account with an address that does not forward externally. If they control the organization's policy, their administrator can allow external forwarding.
Before escalating: confirm that this is not your own domain's problem. An unverified sender can look like external forwarding to some filters. Complete email authorization for your domain, then check whether the response is still returned. If the member's organization is the blocker, Checking the email quarantine on popular email systems explains what their administrator can release and allowlist.
The recipient's organization restricts who can send to them
Provider responses: "550 5.7.129 RestrictedToRecipientsPermission; not authorized to send to recipient", "554 5.7.1 Recipient address rejected", "authentication required; Delivery restriction check failed".
What it means: the recipient's email organization only accepts mail from an approved sender list or requires specific authentication, and our mail is not on it.
How to resolve: the member should ask their email administrator to allow mail from the forum's sending domain, or update their account with a different address (for example a personal address that has no such restriction).
The fix path: complete email authorization for your forum's sending domain first, so the receiving system can verify it with SPF, DKIM, and DMARC. Then ask the member's email administrator to allowlist the sending domain and release the message. Checking the email quarantine on popular email systems has the details to send them, and Why isn't a member receiving our forum emails? (debugging guide) walks through the whole sequence.
The message was rejected as spam or by a content policy
Provider responses: "Message blocked due to spam content", "554 5.7.1 Message rejected due to local policy", "Administrative prohibition - envelope blocked", "554 5.7.9 Message not accepted for policy reasons".
What it means: the recipient's provider or a content filter rejected the message itself, usually because of its content, the sending domain's reputation, or a filter rule on the recipient's side.
How to resolve: the member should check their spam or junk folder for the message and mark it as not spam, then click re-enable email delivery on their Preferences page. If the message was a forum notification, the member can also adjust which emails they receive in their subscription settings.
The domain does not exist
Provider responses: "Domain does not accept mail (nullMX)", "Host or domain name not found. Name service error".
What it means: the domain part of the address (everything after the @) no longer has working mail service. The domain may have expired or been abandoned.
How to resolve: the member must update their account with an address at a domain that can receive mail.
The address was marked as spam by the member
What it means: the member reported one of the forum's emails as spam. The address is then unsubscribed from that type of email and, if unsubscribing fails, marked as invalid.
How to resolve: the member should mark a legitimate forum email as not spam in their spam folder and click re-enable email delivery on their Preferences page, or update their account with a different address.
Temporary failures
Provider responses: "temporarily deferred", "450 4.2.1 mailbox busy", "Message delayed", "452 Insufficient storage".
What it means: the provider rejected the message temporarily and our system will retry it. Temporary failures do not mark the address as invalid.
How to resolve: no action is needed unless the retries keep failing, in which case the responses above will appear instead.
Re-enabling email delivery
The message a member sees on their Preferences page includes a re-enable email delivery link. Clicking it clears the flag so the forum starts sending to the address again. It should only be used after the underlying problem has been fixed (mailbox space freed, forwarding rule removed, address corrected). If the address still cannot receive our email, it will be marked as invalid again, and the new provider response will be shown the next time the member signs in.
Not sure where to start?
If you are not sure which of these responses applies, or the member still is not receiving email after the fix, follow Why isn't a member receiving our forum emails? (debugging guide). It works through the email history, domain authorization, spam folders, and email quarantine in order.