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 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).

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.

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