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ISP requirements for outgoing or return REPLY TO email addresses and perform nslookup to prevent you from sending emails to another ISP SMTP mail server...   

Can I use a non-existing sender FROM: email address to send or broadcast messages?

Sending or Broadcasting emails with a fake or non-existing FROM: email address usually results in bounced (undeliverable) mails and the sender may get an error message like this: (some ISPs simply accept and delete your mails without notification).

 572 Relay not authorized or not local host…
 not a gateway

 550 This address is not allowed or Requested
 action not taken: mailbox unavailable



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572 and 550 RFC delivery status notification: The setting of the From Address are incorrect and/or an attempted was made to deliver but there was a non fatal error and it will be retried and/or some change to the message destination must be made for successful delivery View the list of RFC error return codes

Most ISP mail server have some requirements for outgoing or return REPLY TO email addresses as well as performing a reverse nslookup to prevent and disallow you to send emails to another ISPs SMTP mail server.

If the sender or recipient is a non-local user and to protect the mail server from being used by spammers to relay their messages by using another company’s network and computer resources. See: Blacklist

nslookup is a DNS whois tool that perform forward and reverse DNS queries for the current domain address (to get an IP Address of a hostname and hostname of the IP address).

IP addresses are allocated to companies and ISPs in blocks. Anyone can find out who administers a block of addresses or the owner of a domain address by using an IP Block whois tool to query the Internic database.
IP, DNS Address

Non-local user?
Many ISPs mail server prevent relaying of SPAM by not allowing emails to be sent--that is outside the sender domain, meaning that;
MAIL FROM or REPLY TO is using an email domain that is different from the log-in domain address.

The DNS server IP Address have nothing to do with the process of email message delivery. It merely serves to turn the IP address--a series of four 8 bit numbers, ie: into  (a domain name) that's all. To get a DOS prompt in Windows 2000 or XP,
go to >Start >Run and key in CMD and click OK

DNS (Domain Name System) is a distributed domain based naming system database that can be query to identify administers of IP Address--which is a string of numbers, example: are translated into a string of "letters" called Domain Name  - example: = Domain Name

When we send an email we uses character string to specify a destination, like; 2service @ but the machine on the internet does not understand this name (character string).

The machine uses a protocol called IP (internet protocol) and numbers like these; to represent a destination, which is not very user friendly, therefore, a special internet service called DNS (Domain Name Service) is to help convert character string (name) into numbers.

Every email domain or website URL has their own individual destination number that is different from another.

Therefore, we send an email to your local ISPs mail server, they will also have a local DNS (Domain Name Service) server (sort of like a mirror DNS site) that can be queried, identify and locate the actual destination of a character string (name).

Anyone can use a DNS Program query tool freeware from to access a DNS Name Server on the Internet and gather data in its DNS database.

551 User not local, please try <forward-path> or Invalid Address: Relay request denied

RFC delivery status notification seems to indicates: The recipient's email address have changed and your ISP mail server is forwarding it back to you and/or your ISP SMTP mail server does not accept email when neither the sender nor the recipient is a non local user.

Many ISP filters incoming emails and automatically delete mails where the TO address is different from the actual recipient address-delete not bounced-hence you won't know if the message actually got to the intended recipient. See: Email Filtering

Notes: If you are using your own mail server program See: Mail Server FAQs





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