Failed indicates that we were unable to transmit your message to the delivery provider. This could be due to a number of reasons, such as an invalid email address or spam filter. Try sending another message in a few minutes.
When you receive email from a "MAILER-DAEMON" or a "Mail Delivery Subsystem" with the subject "Failed Delivery" or something similar, it signifies that an email you sent was undeliverable and was "bounced" back to you. The most typical cause of a failed delivery is because the email address you provided is invalid. Sometimes email providers will reject emails that appear to be spam.
Here's what happens when you send an email: Your email program contacts a mail server to send the message out. Most likely this will be the Internet Mail Service (IMS) for your company or organization. IMSs are usually run by an ISP (Internet Service Provider), such as Verizon, Comcast, AT&T, etc. Email servers play a role in making sure messages get delivered. If someone sets up their email server so that it rejects certain types of messages, then you might not receive some emails. This is called "spam filtering" and is done by many servers to protect users from sending valuable information into junk folders.
If you try to send email to an invalid address or one belonging to another user, you'll get a "mailer-daemon failure notice". This means the email wasn't delivered to its intended recipient. It may have been rejected by the recipient's email server for various reasons; sometimes they may be busy and unable to accept emails at this time. Other times they may be aware that you're trying to send them spam and block your emails accordingly.
The message failed indicates that you are unable to iMessage that specific contact for one of several reasons. Their phones may be switched off or have no signal, among other things. They may have migrated to Android without first deactivating iMessage. There are also some issues with iOS 12 that may cause messages to fail to send.
Two responses Messages may have been changed to MMS, necessitating the need of a data connection. Turn off WiFi, make sure your data connection is turned on, and try again. If that doesn't work, you'll need to contact your carrier.
A message might be sent from your end but not received by the receiver. It might be a connectivity issue, a configuration issue, or simply that they disregarded the notification. In any case, it's important to understand why the message was not delivered so you can fix the problem next time around.
It denotes that the message was not successfully delivered. Please try again. It doesn't have to be because of a weak signal. Even in good signal locations, this issue can occur. Congestion may occur on the network if the network is nearing its carrying capacity limit. This will result in the message being delayed.
Have you tried sending the message again after some time? Many messages become unsuccessful because they are sent too early or too late. Try sending your message at around the same time each day. That way, you won't miss any important conversations.
Resolving Mail Delivery Failures: Returning Messages to Senders
In other words, every email generated by the "Mail Delivery Subsystem" ("MAILER DAEMON") comprises the cause for the message delivery failure. But what if you get hundreds or thousands of "Mail Delivery Failed-MAILER-DAEMON" alerts even though you haven't sent any emails? That's because some email server is rejecting your messages as being too large (long message headers).
Here are some common causes for this error:
If you're using a free email service such as Gmail or Yahoo! Mail, you will most likely receive this error message because these services limit the size of incoming email to 10MB. Even if you have not hit that limit, it may be reached by some recipients. To resolve this issue, you will need to send emails with file attachments separately from regular emails.
If you're sending email through an external SMTP server, make sure its maximum message size is within limits set by your own server. Some servers allow up to 100MB in total, while others restrict it to 10MB. If the latter is the case, send only small attachments (<10MB) or none at all.
Finally, if you're sending email using Microsoft Windows Outlook, make sure "Show read messages after they are moved to the Deleted Items folder" is not enabled.