Definition
Email quarantine and rejection describe two distinct ways a receiving server can keep a message from reaching the recipient. A rejection is a hard refusal: the server returns the message to the sender during delivery (a hard bounce), often with a specific error code. A quarantine places the message aside for review or filtering rather than delivering it, without necessarily bouncing it.
Understanding the difference matters because each has different causes, different impact on your reporting, and different remedies.
Rejection vs Quarantine
| Factor | Rejection (Hard Bounce) | Quarantine |
|---|---|---|
| Outcome | Message refused and returned | Message held, not delivered |
| Trigger | Unrecognised user, policy, or server block | Suspicious content or sender signals |
| Reporting | Counted as a bounce | May not appear as delivered or bounced |
| Sender action | Fix address or sending setup | Improve content and sender reputation |
A rejection is usually definitive; a quarantine is a judgement call by the receiving system.
Common Causes of Rejections
- Unknown recipient (550): Mailbox does not exist.
- Policy rejection: Server refuses mail from the sender.
- Content rejection: Message flagged by rules but not necessarily sent to potential recipients.
- Rate limiting: Sender exceeded allowed volume.
How to Handle Each
- Rejections: Remove or correct the bouncing addresses, check authentication, and review sending policy compliance.
- Quarantine: Focus on content quality, sender reputation and engagement; quarantine often clears as reputation improves.
- Monitor both: Track bounces and quarantine-related delays in your deliverability dashboard.
Related Glossary Terms
Abuse Complaint
An abuse complaint is a report from a recipient who marks an email as spam, which negatively affects sender reputation and deliverability.
Alt Text
Alt text is the written alternative to an image in an email, displayed when images are blocked, slow to load, or consumed by screen readers.
AOL Mail for Email Marketers
AOL Mail is a legacy email provider with specific deliverability requirements and rendering quirks, now operating as part of the Yahoo+AOL network under shared infrastructure.
Email Authentication Protocols
Email authentication protocols are technical standards that verify the identity of an email sender, helping mailbox providers distinguish legitimate mail from spam and phishing.
BIMI
BIMI (Brand Indicators for Message Identification) is an email standard that allows brands to display their logo next to their emails in supported email clients. It requires DMARC enforcement at quarantine or reject policy.
Bounce Management
Bounce management is the process of handling emails that are rejected by mailbox providers, including classification, removal of bad addresses and protection of sender reputation.
Frequently Asked Questions
A rejection is a hard refusal where the server returns the message to the sender as a bounce. A quarantine holds the message aside for filtering or review without a definitive refusal, so it may not show in reporting as delivered or bounced.
Rejections commonly result from an unknown recipient address, sender policy violations, content rules, blocked sending domains or rate limits. The receiving server returns a specific error code that identifies the reason.
Usually not clearly. Because the message is held rather than delivered, it may not appear as a successful delivery in your reports, and it can be difficult to detect and attribute.
Strip and correct bouncing addresses for rejections; improve sender reputation, authentication and content quality for quarantine. Tracking both in your deliverability dashboard helps you treat each with the right remedy.