Suspicious whitespace sequence my message

[rebadoc]

I'm using the free version of Avast. I sent an email announcement to a group. One person's copy bounced. When I tried to click on the return message I got an avast alert telling me that I received a "suspicious whitespace sequence" with choices to Delete, Block or Continue. The subject in my original message did have 2 phrases so I put a few spaces between them.

No one else's copy had a problem and I've been sending similar announcements to this person for over a year with no problem.
1. What is wrong?
2. What should I do and how can I prevent this problem?

Please help.

[DavidR]

If an email is received with a large number of spaces (white space) in the subject or an attachment file name it could be because this used to be a tactic employed to hid malicious intent by forcing information out of the visible text window.

So are there large numbers of spaces in the announcements you are sending or do they have attachments with lots of spaces in file name ?
If so you can correct them.

Perhaps no one else is using avast's email scanner which has a heuristic type check for white space amongst other things. It would also depend on how they have the sensitivity set 'High' and if they have customized the Heuristics settings (I have mine set to 5, see image).