The bounced back message below is received by the sender.
"Sender verify failed"
To prevent spamming from any random domain name that does not exist, the mail server is configured to verify all incoming sender mail domain are real and have a valid mail server.
Upon receiving an email, the mail server will try to connect to the remote sender's SMTP server from which the mail originates.
If the sender's server can't be reached, or the sender's email address doesn't exist, the email send will be rejected.
Also if the recipient mail server server cannot find the sender's mail domain MX record, the verification step will fail as well.
The receiving mail server will try to verify that all sender's are valid and the sender is able to receive email when you reply back to them.
This will prevent spamming from any random domain names that does not exist.
Solution: If the sender is valid, the sender's mail admin may need to check their DNS setting to ensure that MX record is available.