There are two parameters configured in main.cf that require strict adherence to Internet email standards. Enable the smtpd_helo_required parameter to require that SMTP clients start the conversation with the HELO/EHLO verb, as described in the SMTP RFC. By default Postfix is rather lenient with clients that do not follow the protocol exactly. If you specify smtpd_helo_required = yes, and a client skips this step, Postfix rejects the message. The RFC also specifies exactly how envelope addresses should be formatted. Normally, Postfix accepts nearly any envelope address that it can make sense of, but if you specify strict_rfc821_envelopes = yes, Postfix rejects messages from clients that do not send correctly formatted addresses.
In actual practice, it's probably a good idea to require HELO because most clients at least follow the basic steps of the protocol. On the other hand, there are a number of clients that don't get address formatting correct. Being too strict here might lose legitimate messages.
General Configuration and Administration
Email and DNS
Local Delivery and POP/IMAP
Hosting Multiple Domains
Blocking Unsolicited Bulk Email
Transport Layer Security
Appendix A. Configuration Parameters
Appendix B. Postfix Commands
Appendix C. Compiling and Installing Postfix
Appendix D. Frequently Asked Questions