When sendmail(1M) reads from anything that might time out, such as an SMTP connection, it sets a timer to the value of the r processing option before reading begins. If the read does not complete before the timer expires, this message appears and reading stops. (Usually this happens during RCPT.) The mail message is then queued for later delivery.
If you see this message often, increase the value of the r processing option in the /etc/mail/sendmail.cf file. If the timer is already set to a large number, look for hardware problems, such as poor network cabling or connections.
For more information about setting the timer, see the section describing the sendmail(1M) configuration options in the System Administration Guide, Volume 3. If you are using AnswerBook online documentation, the term "timeouts" is a good search string.