If you're receiving e-mail messages and there are random exclamation marks (!) at the end of lines, you've likely encountered the odd "line length limit indicator" added by Sendmail MTAs.

The Internet standards for SMTP indicate that a single line of an e-mail message can be no longer than 1000 characters. If sendmail encounters a longer line, it wraps it and adds an exclamation point (!) at the end of the line as an indicator of its behavior.