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Microsoft 365 / WiscMail / WiscMail Plus - Understanding SMTP errors

This information in this document is intended for email administrators or campus mass emailers who are seeing errors when sending email smtp.wiscmail.wisc.edu. If you are a non-Office 365 user and are experiencing these errors, then please contact your email administrator. If you are a Microsoft 365 user and are seeing these errors in your email client please contact the DoIT Help Desk for assistance.

5.7.1 ... must use TLS ...

UW-Madison's Cybersecurity policy requires TLS between mail servers at certain domains. See also: Microsoft 365 - Email Domains that Require TLS. Application admins sending mail to the affected domain should enable TLS in their SMTP client. Remote mail administrators should enable "opportunistic" TLS for UW-Madison's domains; and configure mandatory TLS if opportunistic TLS results in this error.

  • Your primary address and any of your alternate addresses
  • Any address of any account to which you have linked access

It is common for remote mail servers to reject, or mark your messages as spam, if you are sending as an external address (e.g. @gmail.com) if you're not using the authorized SMTP server for that domain. It is strongly recommended that you set up an additional SMTP profile in your client settings so that you may send mail through that provider's SMTP server.

5.7.1 ... Postmaster Address Not Allowed from ...

This error indicates that the message appeared to be sent from postmaster@wisc.edu but did not originate from one of the WiscMail or UW-Madison Office 365 servers. Only WiscMail servers can send as Postmaster.

550 5.1.1 unknown or illegal alias

This error message means that the email address does not exist on the server.

If this error message comes back to you in an email "Delivery Notice" message, it means that your client was able to send the message to your outgoing server, however your outgoing server was not able to deliver the message to the server that hosts the account. Most outgoing servers will deliver the message to the remaining recipients, so you only need to re-send the message to the failed recipient once you have corrected their email address. However, this depends on your outgoing mail server, so it is recommended that you verify this behavior.

If this error message was given to you by your client, it means that your outgoing server was unable to accept the message because of the invalid recipient address. Most clients will abort the entire message and not deliver the message to the remaining recipients, so you should correct the email address of the failed recipient and then re-send the message to all recipients. However, this depends on your client, so it is recommended that you verify this behavior.

Unable to send email

If you receive the following errors/bounce back messages:

  • "The delivery has failed to these recipients or groups: your message couldn't be delivered, your email address is suspected of sending spam and can't send outside of your organization. Please contact your email admin."
  • "The message couldn't be delivered because you weren't recognized as a valid sender. The most common reasons for this is that your email address is expected of sending spam and its no longer able to send messages outside of your organization. Contact your email admin for assistance."

Your account may be blocked by Microsoft. Please contact the DoIT Help Desk to further troubleshoot this error.

Other Errors

  • 550 5.1.8 invalid/host-not-in-DNS return address not allowed (Usually outbound)
  • 550 5.2.4 alias failed to expand to any valid addresses
  • 550 5.7.1 WiscMail error 1279. Invalid from address
  • 550 5.7.1 Bad reputation
  • 552 5.2.3 Message exceeds local size limit
  • 552 5.3.4 a message attachment exceeds local limits
  • 554 5.1.4 Mail hop exceeded possible mail loop

4xx errors vs. 5xx errors

Messages should not be rejected as a result of 4xx error codes. 4xx error codes are temporary errors that are designed to tell the sender to try sending at a later time. The only time that a server should reject a message as a result of 4xx error codes from WiscMail servers is if the sending server decides to stop retrying the messages. In this case, the sending server should reject the message with its own 5xx error code (e.g. 554 Transaction failed).



Keywordsoffice 365 o365 m365 microsoft 365 5.7.1 550 5.2.4 5.1.8 5.7.1 5.2.3 5.1.4 5.5.1 must use tls not allowed to send email postmaster address unknown or illegal alias invalid host return failed bad reputation message exceeds local size limit hop exceeded mail loop service account netid proofpoint   Doc ID3998
OwnerO365 S.GroupMicrosoft 365
Created2005-09-07 18:00:00Updated2023-11-06 07:30:54
SitesDoIT Help Desk, Microsoft 365
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