This is a message from the Heuristic mail analysis of avast.
Right click the 'a' blue icon, click 'Details' buttom, choose Internet Mail at left and Customize buttom at right.
Then go to the two Heuristic tab of settings.
If you use MS Outlook, then you have to use the plugin MS Outlook following the same as above.
The help files will help you: Check whitespaces sequence. Some viruses use a trick: behind one extension of the infected file name a large number of spaces (or other nondisplayable, "white" characters) is appended, followed by a second, real extension that is dangerous. The user does not see the second extension (it is several lines below or does not fit into the window where names are displayed). Heuristic analysis can uncover this trick and warn the user. Default permitted sequence length is five. Thus, if there are more than five white characters, a warning message will be displayed.
