Many company and university mail systems hold suspicious or unverified email in a quarantine instead of delivering it to the inbox. Nothing is bounced, so the sender sees the email as delivered while the recipient never sees it in their inbox or their spam folder. This article explains what quarantine is, where to check it on the most common systems, and what a mail administrator can do to release a held message and stop it happening again.

Why corporate and school systems quarantine forum email

Mail administrators set policies that hold email which fails an authentication check or looks like bulk mail. Forum notifications are bulk email sent to many recipients, so they are checked closely. A message is usually held for one of these reasons.

• The sending domain has incomplete SPF, DKIM, or DMARC authorization, so the receiving system cannot verify that the forum really is allowed to send from that domain.
• The message tripped a bulk sender policy. Google, Microsoft, and other providers require bulk senders to authenticate their domain, keep spam rates low, and include a one-click unsubscribe link.
• A content or reputation filter on the receiving side flagged the message. This can happen even when authorization is complete.

A quarantined message is different from a bounced one. A bounce is rejected outright and appears as a Delivery failed entry with the provider's response in your email history page. A quarantined message is accepted, so the email history shows Sent even though the member never received it. If you are a forum administrator debugging a missing email, start with Why isn't a member receiving our forum emails? (debugging guide).

Microsoft 365 and Outlook

Microsoft 365 holds filtered mail in quarantine, which members and administrators review in the Microsoft Defender portal at protection.microsoft.com.

• Members: sign in to protection.microsoft.com with your work or school account and open the quarantine page under Email & collaboration. Depending on the policy your administrator has set, you can release the message yourself or submit a release request for approval. Also check your Junk Email folder, since some messages are delivered there rather than quarantined.
• Administrators: run a message trace to find the email and see why it was held, then release it from the quarantine page. Release settings live in the quarantine policies under Microsoft Defender for Office 365, and a policy can allow users to release their own messages or reserve that to administrators.
• Rejections instead of quarantine: Microsoft 365 also rejects some forum email outright. If the email history shows a Delivery failed entry with 550 5.7.520 (organization does not allow external forwarding) or 550 5.7.129 (restricted recipients), see Email delivery error messages and how to resolve them for what each response means and how to fix it.

Google Workspace and Gmail

• Members: check the Spam folder first, then open All Mail and search for the sender's address or the message subject. Some held messages are delivered straight to All Mail without appearing in the inbox.
• Administrators: quarantined messages are reviewed in the Google Admin console's Gmail quarantine. A message released from there is delivered to the member's inbox. The Gmail audit log records which messages were quarantined and why, which is the fastest way to confirm that a forum notification was held rather than dropped.
• Bulk sender rules: Google and Yahoo require bulk senders to authenticate their domain with SPF, DKIM, and DMARC alignment, keep their spam rate low, and include a one-click unsubscribe link. Completing email authorization on your forum satisfies the authentication part of that requirement.

Proofpoint, Mimecast, Barracuda, and other filters

Most other corporate filters, including Proofpoint, Mimecast, and Barracuda, follow the same pattern. The member will not be able to see or release the message themselves. Ask them to send their IT administrator this request.

Please check the email quarantine for a message sent to [member's address] from our forum's sending domain, [your forum's outgoing email domain], on [date], with the subject [subject]. Please release it to my inbox, and please allowlist the sending domain so future messages from the forum are delivered directly.

The member's IT administrator can find the message by searching the quarantine queue for the recipient's address or the sending domain. The forum administrator can supply the exact recipient, subject, and date from the email history page.

What the mail administrator can do

Releasing the single message fixes that one email. Allowlisting makes the fix permanent.

1. Find the held message in the quarantine queue or message trace and release it to the member's inbox.
2. Add the forum's sending domain to the organization's safe sender or allowlist list so future messages skip the filter.
3. Check the sending domain's authentication. Email from your domain is signed with the CNAME record websitetoolbox._domainkey, which points to websitetoolbox._domainkey.websitetoolbox.com, and the SPF record authorizes mailers.websitetoolbox.com. Both are published through the email authorization setup page, and completing them is what stops the receiving system from treating the email as unverified in the first place.

Allowlisting works together with completed authorization rather than replacing it. An allowlist entry tells the receiving system to trust your domain; completed SPF, DKIM, and DMARC records are what let every other receiving system verify that trust on its own.