This refers to a general protocol error when SMTP protocols are out of sequence. For example, an SMTP protocol error occurs ...

{4}: This refers to a general protocol error when SMTP protocols are out of sequence. For example, an SMTP protocol error occurs when AUTH is tried before EHLO. In one observation, this occurred when the system was experiencing an out of disk condition. Troubleshooting: Run an SMTP protocol log or a Network Monitor trace and make sure that there is enough disk storage and virtual memory for SMTP to operate.