Exclamation marks being inserted into Emails at approx 1000 character mark

Got hit with this myself.

If you are building a html message without any carriage returns in it, you may get errors somewhere around the 1000 character mark.

Solution is to add a few \r\n characters so that each line is smaller than 1000 (or 988 if you don't count the \r\n characters).

See section 2.1.1. Line Length Limits of RFC 2822 for more details:

2.1.1. Line Length Limits

There are two limits that this standard places on the number of characters in a line. Each line of characters MUST be no more than 998 characters, and SHOULD be no more than 78 characters, excluding the CRLF.