#5.7.1 smtp;554 5.7.1 Unable to verify sender address "This is the error I g
et"

From http://www.developmentnow.com/g/92_...elopmentnow.com

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> Unable to verify sender address

Self-explanatory error, to a degree.  The destination mailserver is
checking the validity of your MAIL FROM address.  The most primitive such
check simply checks if your sender domain exists in the public DNS; a
deeper check checks for an MX record; and the deepest such verification,
called a Sender Address Verification (SAV) callout, actually connects back
to _your_ MX to determine whether your full sender address accepts mail.
SAV checks can be flawed in several common scenarios, but unless you
expect that you _should_ be failing SAV at this time (for example, if your
MX is down), you should keep looking for the root of the problem on your
side.

Do you see a hit in your logs from the remote server back to your MX
during the session (that's what a callout will look like)?  If so, how
does your server respond?

What's the exact MAIL FROM address of these messages?  Can you verify it
at outside DNS servers?

--Sandy

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