When an email can't be delivered, you get an automatic reply called a bounce (often titled "Undelivered Mail Returned to Sender" or "Mail Delivery Failure"). The bounce tells you exactly why it failed. This guide shows how to read one and what to do about each type.
Step 1: Find the error line
Open the bounce and look for a line with a 3-digit code such as 550, 552 or 554, followed by an explanation. For example:
host outlook-com.olc.protection.outlook.com said: 550 5.7.1 Unfortunately, messages from [x.x.x.x] weren't sent… part of their network is on our block list (S3150)
Step 2: Match it below
| The bounce says… | What it means | What to do |
|---|---|---|
| User unknown, does not exist, no such user, address rejected | The recipient's address is wrong or no longer exists. | Check the spelling of the address. Ask the person for their correct email. |
| Mailbox full, over quota, insufficient storage | The recipient's mailbox is full. | Let them know (by phone or another address). If it's your mailbox that's full, see Mailbox full or "over quota". |
| Block list, blocked using Spamhaus, S3150, Service unavailable, client host blocked | The recipient's provider (commonly Outlook.com/Hotmail) is temporarily blocking a range of sending servers. | This is on our side to fix. Open a ticket and attach the full bounce. We work with the provider to remove the block, usually within 24–48 hours. |
| DMARC, SPF, not authorized to send, authentication failed | Your domain's email DNS records are missing or conflicting (for example two DMARC records, or no SPF entry for the service you're sending from). | Check the records in DNS records needed for Liberation Email / Hub for Teams. Make sure there is only one SPF and one DMARC record. |
| Rejected by policy, Barracuda, Mimecast, Proofpoint | The recipient's company firewall rejected the message. | Ask the recipient's IT team to allow (whitelist) your address or domain. |
| Message size exceeds, too large | The attachment is too big. | Share large files as a link from Hub Files instead of attaching them. |
| Greylisted, try again later, temporarily deferred (4xx code) | A temporary delay, not a failure. | Nothing. The server retries automatically. If it still isn't delivered after a day, open a ticket. |
Step 3: Sending us a bounce
If the fix is on our side, or you're not sure, forward the complete bounce email (or attach it) to a new ticket. Please don't just describe it: the technical details at the bottom tell us which server rejected it and why. Also include:
- the address you sent from and the address you sent to,
- whether it happens with every recipient or only some (for example only Hotmail/Outlook addresses).
Still need help?
Open a support ticket and include the details listed above so we can resolve it on the first reply.
