An email bounces back for several reasons. If any of your emails are not delivered, go to your Inbox and read an email named Undelivered Mail Returned to Sender. The reason will be clarified there.

In this article, we list two types of bounced emails to help you solve delivery issues.

Type 1: Emails get rejected by the receiving server

When there is a host server specified in the undelivered email, it means the email had reached the receiving server but somehow got blocked.

Common causes and their solutions are listed as follows:

Low reputation

• Message: 421-4.7.0 suspicious due to the very low reputation of the sending IP address
• Cause: Some essential DNS records have not been set up yet.
• Solution: Make sure you have set up SPF, DKIM, and DMARC records for your domain. These records can help improve your mail server's reputation. If the issue persists, ask your ISP to change the IP address.

Missing Message-IDs

• Message: Messages missing a valid 550 5.7.1 messageId header are not accepted
• Cause: The email does not have a valid Message-ID in the header.
• Solution: Message-IDs are generated by the mail client that sends your emails. Check your mail client's settings for adjustments.

Unknown user

• Message: Recipient address rejected: User unknown in local recipient table
• Cause: The recipient you tried to reach does not exist on the destination mail server.
• Solution: Contact the recipient to check the email address.

Unavailable service (550 5.7.1)

• Message: Service unavailable; client [IP address] blocked using DNSBL server name
• Cause: The sending server's IP address is listed in DNSBL.
• Solution: Go to MXToolBox or Spamhaus to check if your domain is on a block list. If so, contact the list holder to remove your domain. If they refuse to help, you can consider requesting a new IP address from your ISP.

IP missing PTR records

• Message: 550-5.7.25 from IPs with missing PTR records.
• Cause: The sending server's IP address has no PTR records.
• Solution: Perform a reverse DNS lookup. If it fails, contact your ISP to add the appropriate PTR records.

Type 2: The sending server is unable to deliver emails

When there is no host server specified in the undelivered email, it means the sending server has trouble delivering emails.

Common causes and their solutions are listed as follows:

Unknown name or service

• Message: unable to look up host domain name; Name or service not known
• Cause: The requested name or service cannot be found. This might result from DNS issues or spelling errors in the recipient's email address.
• Solution: Check the DNS server at DSM Control Panel > Network > General and see if the recipient domain can be resolved properly. If everything is ready, send the email again later.
  

Timed-out connection

• Message: Connection timed out
• Cause: There is a network connection issue between the sending server and the receiving server.
• Solution: Check the network settings such as outbound ports, firewalls, etc.

Fix email delivery problems

Given the above examples, if your email bounces, open Undelivered Mail Returned to Sender to find the following information:

• Which of the two causes the delivery failure: the sending server or the receiving server?
• What is the reason for the rejection?

With the information, you can either modify your settings or contact the recipients, recipients' IT teams, or the responsible email providers for further assistance.