var/ log/ splunk/ splunkd. SMTP Error 501 : The command was correct and recognised, but the parameters ( the arguments, e. email address) were not valid. refused to talk to me: 501 Syntax error in parameters or arguments. E- Mail is NOT a SECURE channel * * *. for the Postfix smtp( 8) client. Why does mail from a certain remote mail server always reject with the error message: ' 501 Syntax error in parameters or arguments' and what.