It is an ASCII linefeed (LF) character not preceded by an ASCII carriage-return (CR) character.

Every line in an Internet mail message is required to end with CR LF. The entire message ends with CR LF dot CR LF. 822bis specifically prohibits other uses of LF.

The mail clients discussed above are incorrectly ending lines with LF and, in most cases, ending the entire message with LF dot LF. That's not CR LF dot CR LF, so a server such as msn.com will sit there waiting for the rest of the message. After a while it'll give up and drop the connection. Your mail doesn't get through.

Some mail servers convert a bare LF into CR LF, and accept LF dot LF as the end of a mail message. This behavior is specifically prohibited by 821bis.