Random “!” in html emails

I have a system written in PHP, running on a UNIX apache server, which generates html emails as part of its functionality.

I have found that an exclamation mark randomly appears somewhere in the email about once per 5 emails generated.

I recently noticed the same behaviour in a Perl/CGI script.

The possibilities I have considered are:
– Bug in Unix mail application when creating html emails
– Call to mail application needs its stream to be flushed first.
– Something else…?
Any help with this would be greatly appreciated.