Non-Deliverable Error Messages


Exploring Undeliverable Mail Messages (Bounces)

Below is a list of common errors reported when an email is returned to your mailbox:

  1. Error 550, User unknown, Mailbox not found, or Recipient does not exist --The person you are trying to send to no longer has an address by that name or the email address is misspelled. If you get this error, double check your spelling of the email address and confirm that the recipient account is still active. If someone tells you that they are getting the above error message when emailing you, make sure they have the correct spelling of your email address.

  2. The recipient's mailbox is full -- The person you are sending to has too many messages waiting for them and has exceeded their mailbox size limit. If someone tells you that they are getting the above error message when emailing you, go to and clear unwanted messages from the server.

  3. 501 Domain must resolve, Delivery to the following recipients has been delayed --'s mail servers are rejecting the email because of misconfigured DNS settings on the sender's mail server. Click here on how to fix the issue.

  4. The attachment exceeds server limit -- This occurs when the file attachment you are sending is too large for the mailbox or the server does not allow messages to be over a certain limit. Hotmail is an example of a server that does not allow attachments over 2 megabytes. recommends only sending files 1 megabyte or smaller to minimize upload and download time.

  5. Transaction timed out or Message could not be sent after "X" attempts -- This usually occurs when the recipient's mail server is not available due to network traffic or other issues. Try sending the message again later.

  6. Host not found, Unknown Host, The system cannot find the path specified, or Invalid mailbox-- Caused by misspelling the domain part of the email (ex. spelled, @toast) or if the recipient's mail service is or available (due to an outage, maintenance, etc.).

  7. '' not a gateway or 501: Relaying denied -- This is caused by an invalid configuration of your email program. Click here for correct settings for your email program.

  8. The pipe has ended or Server received Winsock error Connection timed out -- These errors can stem from various problems including: Mail program configuration problem, the host is not found (see #5), or the mail has been filtered and rejected.

  9. 550 <ip address> does not pass SPF requirements for domain. checks all servers delivering email to for SPF requirements. If the IP address of the server delivering email does not match the SPF requirements for the domain email is rejected. Click here on how to fix the issue..

  10. 550 5.7.1 This system is configured to reject mail from [email server] (Host blacklisted - Found on Realtime Black List server ['', '', etc.]) -- may be blocking the message because it's coming from a blacklisted server. Click here on how to fix the issue.

Add Feedback