Re: 451 Could not complete sender verify callout

[Matt Simpson <net-qmlist@jmatt.net>]

At 9:57 AM 1/30/07, Phil Breskey wrote:
> Hi, I am having trouble mailing a few domains out there on the web. 
> When we try to send them an email, it will stay in the queue for a 
> few days, then come back to us with the error "451 Could not 
> complete sender verify callout".
Just guessing, "sender verify callout" sounds like the receiving 
sender is trying to "verify" the sender in some way, and it's not 
working.
For spam control, some smtp receivers try to contact the domain in 
the sender address and validate that the sender exists.  In some 
cases, they will actually attempt to start a new SMTP transaction, 
with a RCPT TO pointing to the address of the sender.  If they 
receive an OK to that, they drop their "verification" session and 
accept the inbound mail.  If they don't receive an OK, they will 
refuse to accept the inbound mail, either temporarily or permanently.
If you look at your smtp logs, you may see inbound connections from a 
krudmart.com host whenever you try to send them a message.  That's 
assuming that the messages you're sending have your domain in the 
sender address.  If you're sending messages with a different domain 
in the sender, then maybe that domain's inbound SMTP server isn't 
providing a satisfactory response to krudmart's verification attempts.