When you attempt to send mail to someone the recipient never receives the mail. The mail is eventually bounced back to you with an error similiar to those below
550 Could not complete sender verify callout
550 Sender verify failed
Some mail servers attempt to validate the sending email address before accepting any mail. They do this by issuing a RCPT TO command to the sending mail server with the senders email address. If the sending mail server responds with a 250 command then they know the sending email address exists and therefore accept the message.
If they are unable to verify the sender then they issue a response similiar to that above.
The main reason that the sender verify can fail is because the remote mail server does not conform to standard RFCs. They issue a RCPT TO command to our server but do not wait long enough for a response. RFC 2821 #188.8.131.52 states that the mail server should wait up to 5 minutes for a response. As part of our email harvesting protection system we sometimes perform a delay on incoming connections.
Contact the recipient by other means and ask them to whitelist your address.
ask the recipient to ensure their mail server complies with RFC 2821 #184.108.40.206.