Apparently when your line length exceeds 998 characters an exclamtion mark is placed in your mail. This is exactly what happend to us when we tried sending a hmtl newsletter using simple news and mime mail module.
When using thunderbird you can see the source of your email. (with other clients probably too) You ll see the html part of the multipart mail is one line. Which offcourse exceeds the line length. Resulting in many Exclamation marks in your mail.
You ll have to chunk up your email before it reaches the mail() function.