Well, this might be serious. It may mean that messages you send are not getting to their destination. Are you able to check independently if your messages do arrive at their destination?

OTOH it could just be a warning that a setting in Thunderbird is asking servers for notification about delivery (Delivery Status Notification) and what they're trying to tell you is that they don't do that. Most servers do not honour DSN requests.

In a message composition window, DSN appears in the Options item in the traditional menu across the top. See if you have it enabled here. If it is enabled and you clear it, then I suspect you won't get any more messages.

But it is not clear (to me) if those warnings mean your messages are not being delivered. I don't know if "failure" means "failed to deliver" or if it means "failed to perform notification" (usually because servers refuse to respond to notification requests).

Hi Zenos Thanks for replying to my question, I do experience messages not being responded to which I always put down to just one of those things. However now I know about "Delivery Status Notification Failure" it makes me wonder if there is something more serious going off. I am unable to check independently if these messages are going through, so I'm stuck. As Gmail is my second email address I can always use my main email address whilst I try and fix this. I have looked under options and it isn't ticked so I assume this means it isn't enabled. Would you recommend I delete my Thunderbird email and then reinstall it ? Would this even help ?

It would help to know if these notifications are for messages you sent or ones that you did not send.

Hi Airmail it's when I start my email up (Thunderbird) I get two email services come through, one via pop 3 and the other gmail. It's messages that come through gmail that these "Delivery Status Notification Failure" appear every day or when I start my email up. I hope this is clear if you need to know anything else please do contact me again.

No it is not clear. DNS messages usually include the info for the message that was or was not delivered. Is this info for messages that you sent or for messages that you did no send?

Post a screenshot of the message.

As a footnote to that last reply:

Spammers commonly forge the sender address, either to get recipients to open the message or just to avoid identification. If they use your email address as their sender or reply address, you will get DSN messages for failed deliveries for a variety of reasons, including nonexistent addresses and spam filter rejections.

Thus, if you see these are not messages you sent, it may be a spam issue, which most commonly is out in cyberspace, but in some cases is an infection on your system or elsewhere within your network.

Hi all in answer to the question of whether the emails are ones I send or receive, it's ones I receive. Mostly it's through gmail but now messages I send (I think) through Thunderbird (pop 3) are not reaching their destination as I'm not getting any replies. Please can anyone tell me how to fix this as I'm concerned that hackers will get my information for sure if this goes on much longer.

Still no answer.

We know that you are receiving the DNS messages. DNS messages are notification a that messages sent by you or with your email address were not delivered.

Question: Did you send the messages referred to in the DNS notification? Are the messages that the DNS notification refers to in your Send folder?

Hi Airmail

Thanks for replying, in answer to your question no I didn't send the messages I'm just getting emails sent to me with the "delivery status notification failure". The messages that have this notification on them are not in my send folder, so really I've got no clue as to where they are coming from. Would it help if I deleted this email address then maybe re-installed it, if that is possible ?

It would be useful to inspect the headers of the offending message. Some servers that raise DSN messages will include header information from the original message.

It will, in all likelihood, show that the messages was sent by someone else, with your name or address, or both, inserted into the "from:" field. There is little you can do about this. The clue to this is the list of IP addresses of the various servers that have handled the message. Whilst these can be spoofed, they are often not, and almost always show the origin to be in some far-flung corner of the world.

Uninstalling, re-installing, whatever, has no influence whatsoever on the fact that someone else knows and mis-uses your email address.

Read this... https://luxsci.com/blog/bounce-back-backscatter-spam-who-stole-my-email-address.html