Some sort of download but do not have app to read. What are you using to send file?
I just noticed that you never got an answer to this.
By 'download' I am assuming you mean the attachments or attachment that was sent with the non delivery report (also known as a 'bounce message') from Virgin Media. In other words VM have automatically generated an email telling you that it has been unable to deliver an email sent from your VM email account. The text of the mailer-daemon email will give you the title of that undelivered email.
The first thing to say is that if the bounce message refers to an email that you never sent it usually means that someone has hacked your email account and is using it to send spam. If that is the case you need to follow all the instructions here:
However, assuming that it relates to an email that you did send then the truth is that there is no need for you to open the attachments unless you are an email guru or geek. The key information is all in the body of the email message itself, as I am sure you have already seen. Usually, as in the case below it's because of "User unknown" which translates to 'you've got the email address wrong'.
The attachments contain a copy of the "email headers" which basically set out the "From", "To" and "Date" information along with the text from your original email. It also includes the routing and timestamps of your email as it traveled from your device to the VM email servers and then across the internet to its intended destination.
That routing information can be useful to email gurus to determine what went wrong. However, in most cases it is down to the fact that the email address does not exist and no further investigation is needed.
Hope that helps.
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