Mailer-daemon messages from Yahoo mean that an email being sent to or from a Yahoo.com email address have encountered an error that prevented them from reaching the intended address. You could be receiving a mailer-daemon email because of things as simple as a typographical error to things as detrimental as a compromised account. The purpose of the mailer-daemon message is the similar to a "receipt confirmation," but tells you something has gone wrong instead of right.
The mailer-daemon message can tell you if you sent an email to an address that doesn't exist, which indicates that you or the person who setup an email link may have made a typographical error when entering the email address. These messages also come from typing in the wrong name, for example, typing Jonathan.Doe@yahoo.com instead of John.Doe@yahoo.com.
Address No Longer Exists
You may receive a mailer-daemon message from Yahoo because an address you sent an email to no longer exists. The address could be typed in correctly, but if the recipient has deleted or deactivated his account they can no longer receive email at the address without activating forwarding to a different address.
Email is Full
While Yahoo Mail offers unlimited storage, the recipient's email service may not. Additionally, a recipient may be forwarding email from Yahoo to another service that may not support unlimited storage. You may receive a mailer-daemon message from Yahoo saying "over-quota" because somewhere down the line an email account is at its capacity.
Message is too Big
Yahoo email supports a maximum attachment file size that is increased over time. If you are sending an email to a recipient that has a smaller maximum attachment size you may receive a mailer-daemon message from Yahoo. You may also receive a mailer-daemon message from Yahoo if you are sending a message to a Yahoo account that is over the maximum size.
A sever error when routing the email from one address to the other can produce a mailer-daemon message. These problems can be caused by anything from a glitch to a temporary server outage.
If you are receiving dozens to hundreds of mailer-daemon messages, particularly for emails that you did not send, it is an indication that your account may be compromised. email spammers may obtain passwords for accounts that don't belong to them and use them to send thousands of emails to any address they can find or generate. Yahoo recommends that if this happens to you, you should immediately change your account password to reclaim your account.
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