OTTAWA – So what exactly makes it OK to use taxpayers’ money to pay for one piece of mail from an MP, but not for another?

The NDP is asking that very question as it fights a ruling that says the party broke the rules governing the use of parliamentary resources — a finding that could end up costing the party and some MPs hundreds of thousands of dollars.

The answer lies within a complex set of House of Commons bylaws that draws a very fine line between acceptable and just too partisan.

For example, Conservative whip and Vancouver MP John Duncan sent out a piece of mail that features a picture of Tom Mulcair, saying the NDP proposes “billions in reckless additional spending, to be paid for by new taxes and higher debt.” Recipients are asked to identify which party is “on track for jobs, growth and long-term prosperity” — a preferred Tory slogan.

That particular piece of mail passed the Commons smell test.

Another, from former Liberal MP Bob Rae, targets the Conservative record on employment insurance, and also includes a survey allowing recipients to choose which party they like best on the issue.

“No region of the country should be punished because of the nature of its employment,” the mailout reads. “Yet that is exactly what Prime Minister Stephen Harper and his Conservative government have done.”

That, too, was considered OK.

But 23 NDP MPs apparently crossed the line with a series of letters, sent in sealed House of Commons envelopes to 26 different ridings, that included the party website and in at least one case the message, “By the registered agent for the NDP.”

Those letters broke the rules, the secretive, all-party committee known as the Board of Internal Economy declared Monday.

An analysis provided by the House of Commons administration on one such letter sheds a bit of light on what’s considered acceptable and what’s not.

One of the disallowed mailings, from NDP MP Guy Caron, was sent into the Quebec riding of Bourassa prior to a byelection there last November. It makes mention of the website.

“Citizens now have an important choice to make,” the letter reads. “They must decide what type of representative they want to send to Ottawa to defend their interests.”

The analysis says that taxpayer-funded mail cannot refer to an election or anything electoral in nature. It must also not direct readers to a party website or any other site that might later solicit donations. Caron’s letter did both.

“In short, it would appear that the mailings were not messages from the individual members as members, but were prepared by and for the benefit of the NDP as a political party and to advance electoral purposes,” reads the Commons analysis.

The highly partisan piece of mail sent from John Duncan’s riding, for example, did not refer to any party website or any future election.

Mail can refer to an MP’s “parliamentary functions,” which include duties and activities that might be performed in a “partisan manner.” In other words, expressing a political position on an issue before the House would be considered acceptable.

Also apparently acceptable is the practice that all parties have of collecting information from different citizens on their political views and preferences — perhaps to be used to track potential supporters.

The NDP decried the committee’s decision Tuesday as arbitrary, describing it as the work of rival Conservatives and Liberals on the committee ganging up on the official Opposition.

“Conservatives attack Tom Mulcair? A-OK! Liberals attack NDP’s patriotism? Looks great! NDP promote healthier environment? How dare they!” said an NDP news release that included a number of examples of partisan mail.

One problem for the NDP revolves around how they orchestrated the bulk mail.

Instead of having the House of Commons services verify the content and then print the material, they had a third-party printer do the work.

The envelopes were then stuffed and sent out using the free mailing privileges afforded MPs — a practice known as franking. No one in any official capacity was ever made aware of the contents beforehand.

The NDP, for its part, insists that it had sought the opinion of the House of Commons administration, and was told that “no rules” applied to sealed mail — although they do apply to other types of pamphlets.