Problem: You recieve the error mesage "451 Could not complete sender verify callout".

Reason: The 451 message can mean a number of different things, but when it comes to the mail and the call outs it relates to one of two things;

1) The sending server denied the requst to verify the senders address
2) The sending server had a problem and could not verify the account.

SMTP Reply Code 451 actually states "Requested action aborted: local error in processing"

What happens in the sender verify is the following;

Our server receives an email from "you@address1.com". Our server then puts that message into a holding queue, and sends a VERIFY ACCOUNT message to the sender to make sure that the email address actually exists. If the address exists, then it delivers the message. If the address doesn't exist (or the server refuses to acknowledge the request) then our server flags the message as potential spam and refuses to deliver the message.

The reason that our servers are setup with the "sender verify" option is because it drastically reduces the amount of spam received. How many times have you recieved an email from "asdfse@hotmail.com" only be find that it's an advertisement for Viagra, mortgages or the latest stock pick. With our verification system, it asks "hotmail" to verify if that address is real and active. If not, it deletes the message and doesn't send it to our client.

By using this system, we reduced the amount of spam on our servers from just over 19,000 emails per day to less than 2,000 emails. It doesn't catch ALL spam, but it blocks a good majority of it.

To help explain what happens, here a simple break down;

Address A gets sent to address B. Address B sends a request back to address A to find out if it's a valid address. Address A acknowledges the reqeust and the message gets delivered. If address A fails, the message is removed.

A to B (send message)
B to A (verify message)
A to B (deliver message)

The reason that the sender verify is failing is because the remote mail server does not conform to the standandard RFCs (in particular RFC 2821) If your ISP or your HOST does not support the verification of the email address, they have setup a server that does not comply to the standards on the web, and they will need to address it (and fix it).

Solution: The 451 code is not something that Hands-on Web Hosting can fix, this is something for the "sending email address" administrator to fix. Contact the recepient by other means and ask them to ensure that their mail server complies with RFC 2821. If they are not the administrator, they will need to contact their administrator and ask them to review the article.  The admin may also want to check the domain against http://dnsstuff.com and http://dnsreport.com (or other sites like it) as they will try to see where the sending domain is not RFC compliant.