White spaces are often used in either subject or attachments to try and disguise the true content. There really shouldn't be a legitimate reason to do it so it is flagged as suspicious, the one thing about it is at least it wasn't detected as a virus.
If you don't recognise who it came form and can't confirm who it came from (contact them by another means) then the easiest and safest thing to do is delete it.
Open the Internet Mail on-access interface and click the Customize button, Heuristics to see what is checked. See the avast! help file, Resident Protection, Internet Mail, Heuristic Page.
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.