the secure:channel

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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.

I couldn’t find any good reason for this behavior. I would expect Sendmail to simply reject the e-mail message instead. I couldn’t find an RFC or Internet standard that indicated that an line length limit indicator was required.

To resolve the problem, you should probably ensure that your not sending an e-mail message with lines over 1000 characters. Try inserting a line break every once in awhile.