According to the SMTP protocol, several SMTP commands can be placed in one network packet and “pipelined”. For example, whenever an email with a CC: header is sent several SMTP “RCPT TO:” commands may be placed in a single packet instead of one packet per command. The SMTP protocol actually requires that errors are checked and synchronization occurs at certain points. However, many spammers will send out everything in a single packet since they are least bothered by the errors and this method is more efficient. Some MTAs, which detect this pipelining, will reject such mail anyways.

You may also like to read:
Enforcing Simple Mail Standards
HELO/EHLO Checking Of Connections
Greylisting of Incoming Messages
Implementing Greeting Delay
No Use In Responding To Spam
Keystroke logging: Types of Keystroke loggers