SMTP Error Messages And Their Meanings
mail server permanently rejected message, If your message contains an Executable attachment, please package before mailing(zip, tar, gz etc). (#5.3.0)
This is a permanent failure. Now matter how many times you try to send this message the mail server will never accept it for delivery. If you have any attachments please verify that they are not on the following list of prohibited executable attachments.
ba,chm,exe,hlp,hta,pif,reg,scr,shs,vbe,vbs,wsf,wsh,dbx,eml,mht,wab
451 Listed on SBL: abc.com
This is a temporary failure, which means that you should try back later. According to the http://spamhaus.org blacklist, you are attempting to connect from a server that is known to house notorious spammers. Please see the URL mentioned for information on how to be removed from the Spamhaus blacklist, at which time you may try to send again.
451 This mail was handled by an open relay - please visit abc.com
This is a temporary failure, which means that you should try back later. According to http://ordb.org, you are attempting to send mail that has passed through an open relay, which means that it allows anyone to send mail from anyone, to anyone, with no form of verfification. Please see the URL mentioned to learn how to fix this, and then retry your mail later.
451 Bad reverse DNS:
This is a temporary failure indicating that you are attempting to connect from a host that does not have reverse DNS. We require that hosts sending mail through our network have reverse DNS records. Please contact your network administrator or connection provider in order to rectify this situation, as all responsible networks should have reverse DNS records.