Greetings Marius;
Doing some searching around it seems this is a return email server message that is being displayed to you.
There are several different options.
1) ISP has issue on their server sending the users email. Possibly caused by the movement of their domain from one server to another, or another server related error. Search the internet for this message and read the posts about sendmail configuration and domain transfer.
2) Corrupt user profile, if the user has Outlook. One solutions I found mentioned creating a new user profile for outlook and this fixed the error.
I would hope that someone with more experience with the Norton error messages may contribute, but again, this is a standard mail server error message.
Regards