Fix "Undeliverable" error when sending email

Applies to: sending email from a Nexus365 account using Outlook desktop or web

Symptoms of a permissions error

When you send an email you receive a new message from System Administrator with "Undeliverable" added to your subject line. The message body includes the text

Your message did not reach some or all of the intended recipients.

      Subject:	...
      Sent:	...

The following recipient(s) cannot be reached:

This message could not be sent. Try sending the message again later, or
contact your network administrator. You do not have the permission to
send the message on behalf of the specified user. 

If you see this error message then solutions 1 and 2 below are most likely to sort this out for you.

Symptoms of an invalid recipient address

When you send an email you receive a new message from Microsoft Outlook with "Undeliverable" added to your subject line. The message body includes the text

Delivery has failed to these recipients or groups:
Examine the text below for diagnostic information returned from the remote server. If
you have further problems then please contact

The following organisation rejected your message: ...

Diagnostic information for administrators:
Generating server: ...
#550-RPT-FAL: Recipient failed verification. 550 Cannot deliver to this recipient. ##

This error message indicates that you have used an invalid or unrecognised email address in To, CC or BCC field of your email. Solution 3 below is most likely to sort this out for you.

Symptoms of a security block

When you send an email you receive a new message from System Administrator with "Undeliverable" added to your subject line. The message body includes the text

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This
is a permanent error. The following address(es) failed:

    all hosts for '[DOMAIN]' have been failing for a long time (and retry time not reached) 

If you see this error message then solution 4 below will enable you to confirm if this is caused by a University security block and provides guidance on how to proceed.

Solution 1 - Preferred email address

This solution is probably the best one if you have email addresses for several colleges or departments and want to change which email address is used on your outgoing mail.

  1. Login to your Nexus mailbox settings
  2. Select your personal mailbox, select Preferred email address settings, and select Submit
  3. Select the email address you want to send from, then select Submit

Solution 2 - Delegated access

This solution is probably the best one if you need to send email from another mailbox, such as a generic mailbox like

  1. Contact the owner of the mailbox or your local ITSS and request permission to send from the mailbox
  2. Write a message in Outlook, and select Options > From (Outlook app) or ... > Show From (Outlook web) to add a drop-down field for selecting the mailbox your message will be sent from

Solution 3 - Check recipient address(es)

  1. First off, note that your email will have been sent to any recipients not listed in the error message. So you don't need to resend your message to them
  2. Double-check that you have typed all recipient email addresses correctly, and refer back to where you found the email addresses originally. Note that even if this was in the To or Cc field of a message sent to you, the original sender may have typed it incorrectly and also got an error
  3. Contact the intended recipient by another means and ask them to email you so that you can reply and/or add them to your contacts list

Solution 4 - Check if destination is blocked for security reasons

This solution requires some basic technical expertise. Consult your local IT team if you need assistance.

  1. Carry out a DNS A/AAAA-record lookup for the domain of the email address you are sending to
  2. If the results comprise some or all of the following IP addresses then it is likely that a university security block is in place for the whole domain:,,, 2a08:600::80, 2a08:600:ff::80

Security blocks are normally put in place where a website is found to be hosting malware. The blocks affect all DNS records including mail server (MX) records, thereby preventing access to websites and mail servers for the domain.

If you are confident that the destination is owned and controlled by a legitimate organisation then it may be worth contacting OxCERT to confirm that the block is still required.

You will not be able to send email from University accounts to the desired address while a block is in place and will need to use telephone, post or other means of communication.

Further information

The preferred email address for a mailbox can only be set in Nexus mailbox settings and is the only address that can used to send email from the selected mailbox. If you configure Outlook to send from this mailbox using any other address then you will get the error message above. In practice this means that you cannot change the address you send from on a message-by-message basis. For example, if you have a college email address and a department email address then you can only send from the address you have selected as your preferred email address, even though both addresses work for receiving email.

Microsoft provide details on how to open and use a shared mailbox in Outlook.

Get support

If you cannot find the solution you need here then we have other ways to get IT support

Get IT support