This list provides detail on the most common NDRs received by NHSmail users.

• • 550 Requested action not taken: mailbox unavailable
  • 550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found
  • 5.1.1 smtp; 550 5.1.1 ’email address’: Recipient address rejected: aol.com
  • 5.7.0 smtp; 530 5.7.0 ‘sender’s email’: Sender address rejected: Must use TLS
  • 554 5.7.1 ‘recipient address’: Recipient address rejected: RBL
  • 5.4.4 X-Postfix; Host or domain name not found. Name service error for name=’domain’ type=A: Host not found

• • 5.0.0 smtp; 550 Unknown user
  • 4.4.1 X-Postfix; connect to domain[IP]:25: Connection refused
  • ‘Delivery to these recipients or groups is complete, but no delivery notification was sent by the destination server’
  • Host server[IP] said: 450 4.1.8 ’email@domain’: Sender address rejected: Domain not found (in reply to RCPT TO command)

550 Requested action not taken: mailbox unavailable

This error message translates to there either being a problem with the recipient’s account or that their mailbox is full or has been temporarily suspended.

Please double check the email address to make sure you have the right address, wait a while and try again. If it still fails, try contacting the recipient through another means to confirm their email address and that their account is active.

550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found

There is a bug in Outlook in Office 365, for which the solution can be accessed on the following link: KB2784785

5.1.1 smtp; 550 5.1.1 ’email address’: Recipient address rejected: aol.com

This error suggests that the recipient’s account does not exist. Please double check the email address for any typos or contact the recipient through another channel and confirm.

5.7.0 smtp; 530 5.7.0 ‘sender’s email’: Sender address rejected: Must use TLS

There has been a recent change in the NHSmail Relay where all email sent from/to secure domains must use TLS encryption. See the following link for more information: Secure Email Standard

554 5.7.1 ‘recipient address’: Recipient address rejected: RBL

This is caused by the connecting IP having a bad reputation according to the noted block list in the NDR. As a consequence, the emails from this IP are being rejected. In order to request de-listing, the owner of the IP needs to contact the service mentioned and submit a request.

If you are experiencing issues with your sending IP(s) being incorrectly added to a deny list, you can request your sending IP(s) are delisted by the corresponding service. Once added, no further occurrences of the IP(s) being incorrectly added to the deny list should occur.

Please note: We currently use several deny lists for securing the service. Using the deny list checker on MxToolBox (Blacklist Check) is the easiest way to determine the deny list you are being noted by.

5.4.4 X-Postfix; Host or domain name not found. Name service error for name=’domain’ type=A: Host not found

This is the result of there not being a MX record for the recipient’s domain. As a consequence emails cannot be delivered.

5.0.0 smtp; 550 Unknown user

This message means that that recipient’s email address simply does not exist. Please double check the email address for any typos or contact the recipient through another channel and confirm.

4.4.1 X-Postfix; connect to domain[IP]:25: Connection refused

This message points to the recipient’s domain refusing connection from the relay. The user/recipient needs to contact the team that supports the recipient’s domain and request them to allow connection from the relay.

‘Delivery to these recipients or groups is complete, but no delivery notification was sent by the destination server’

As the NDR reads the emails were delivered successfully to the recipient, however, the recipient’s server is not configured to send a delivery notification back to the NHSmail relay. This is something that the team that supports the recipient’s domain needs to implement.

Host server[IP] said: 450 4.1.8 ’email@domain’: Sender address rejected: Domain not found (in reply to RCPT TO command)

This message points to an issue with the sender’s MX record. The recipient needs to contact the sender and ask them to their local Support team investigate.

554 5.7.1 <recipient’s email> Recipient address rejected: BLOCK_SEND_ER

This error message suggests that either the sender or the sending domain is blocked on our relay due to being reported by as malicious.
Sender should contact helpdesk on 0333 200 1133 or helpdesk@nhs.net for further investigation.