Customs has refused to answer questions about the "secrets for brownie points" email in part to protect the privacy of the senior executive who was asking for information to give the FBI.
Customs says the public naming of its intelligence chief Greg Davis means the email he sent cannot be publicly responded to.
An email from Mr Davis was released under the Official Information Act in which he told colleagues they stood to benefit if they passed on information about internet tycoon Kim Dotcom.
He told Immigration NZ's intelligence staff "the FBI would be interested in anything we have on Kim Dotcom so any information we can proactively feed to them on him will buy you many brownie points".
The email was sent in September 2011. At the time, no New Zealand agency had received a formal request for assistance from the US in the Dotcom case.
Mr Davis' name and email emerged in a document was released by Immigration NZ after a complaint to the Ombudsman about information being withheld.
The document should have been released earlier but was "incorrectly considered out of scope", said Immigration's parent department, the Ministry of Business, Innovation and Employment.
Questions from the Herald seeking information about whether Mr Davis' was inside Customs' rules have been rebuffed.
A spokeswoman said: "Customs is not able to comment about a named employee's conduct."
Customs also refused to say if any information about Mr Dotcom had been passed to the FBI - again citing privacy.
The email has brought calls from Green Customs spokesman Steffan Browning for explanations.
"There needs to be some very clear explanations. Greg Davis should be held accountable. From that email, that appears to be something that is potentially even illegal that is being suggested."
The refusal by Customs closes the door on any explanation after Customs Minister Maurice Williamson said the email was an "operational" issue and so had nothing to do with him.
His spokesman referred questions to Customs.
At the time of sending the email, Mr Davis was running Customs' Integrated Targeting Operations Centre.
The centre collects huge amounts of personal information about travellers which Customs is empowered under certain circumstances to pass to other countries.
Just weeks before the email was sent, Prime Minister John Key opened ITOC saying: "Anyone who is innocent has nothing to fear."