A bare linefeed is a single linefeed character (LF or ASCII 10) that isn't preceded by an ASCII carriage return (CR or ASCII 13) character. Since a carriage return is always supposed to be followed by a linefeed according to RFC 822bis, any message that has a bare linefeed in it is supposed to be rejected by the email server it was sent to.
When this happens, the receiving email server typically prepares a rejection message that mentions the problem. This is often accompanied with the directive See http://pobox.com/~djb/docs/smtplf.html.
The URL describes the bare linefeed issue and has some suggested fixes for a number of popular Unix-based mailers (and even a few Windows mailers). Unfortunately, there is no fix listed in the event this problem crops up when you're using Exchange Server.
When you're running Exchange Server, the culprit is almost always not Exchange Server itself but a third-party add-on or appliance -- a virus scanner that runs as part of Exchange Server or a hardware device that scans emails in transit to and from your Exchange server.
Under certain conditions (such as a very long, unwrapped text line), the virus scanner or appliance may mistakenly inject a bare linefeed and cause outgoing email to fail. Some more archaic email clients will also try to end a message with a linefeed, a period and a linefeed instead of a CR-LF-period-CR-LF string, which will also fail.
The easiest way to find out if the device or scanning engine in question is the culprit is simply to take it out of service for a time and see if the problem repeats itself. If not, you'll need to see if an upgrade for that device/program exists -- or perhaps consider a replacement that doesn't cause such issues.
About the author: Serdar Yegulalp is editor of Windows Insight, a newsletter devoted to hints, tips, tricks, news and goodies for all flavors of Windows users.
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