This is a sendmail(1M) message that appears on the console and in the log file /var/adm/messages. If this message occurs once for a particular user, a mail message from this user might end with a partial line (having no terminating newline character). If this message appears frequently or at busy times, especially along with other networking errors, it could indicate network problems.
Check the user's mail spool file to see if a message ends without a newline character. If so, talk with the user and determine how to prevent the problem from occurring again. If these messages are the result of network problems, you could try moving the mail spool directory to another machine with a faster network interface.
During the SMTP receipt of DATA phase, a message-terminating period on a line of its own never arrived. sendmail(1M) timed out and produced this error.