I like to think I’m pretty good at spotting — and immediately tossing — junk mail. My BS detector goes off whenever I encounter letters marked “URGENT” or “Time-sensitive material enclosed” or “Final Notice.”

Occasionally, an envelope arrives that I’m fairly sure is an ad, but I open anyway just to be sure. What if it’s a public notice from the city, telling me someone down the block wants to open a halfway house for criminally insane sex offenders?

The other day I fell for a doozy. The white envelop was blank, with see-through windows showing my name and address and a sender of “SAINT PAUL SERVICE CENTER.” It would’ve been very hard to toss this one without opening it first.

Of course, once I scanned the letter, I could tell it was a mortgage refinancing solicitation. The fine print comes out and states “This is not a government form.”

But at a glance, it wasn’t that easy to discern I was looking at an ad. My bet is that a lot of seniors, recent immigrants and others would believe a government agency had sent the letter.

Here’s the letter. You can click and zoom for a larger version.

The ad is from Midwest Mortgage Capital in St. Louis. The solicitation itself was prepared by Atlanta-based Camber Marketing Group, which specializes in direct mail marketing for the mortgage industry.

I left a message at Camber Marketing, explaining why I was calling, but no one returned the call. I also called Jerry Frost, president of Midwest Mortgage Capital. To his credit, he returned my call. Of course, he defended the ad.

“It’s not deceptive at all,” Frost said. “If you read the piece, it’s clear it’s a mortgage solicitation.”

Frost said the vast majority of people who respond to the letter know they’re calling a mortgage company. “We’re certainly not interested in tricking anybody,” he said.

Frost said consumers who call the number listed in the letter learn right away they’ve called a mortgage company. It’s true. I called and after a brief stint on hold, asked the operator, “Who am I calling?” He told me straight up I’d called a mortgage company.

Frost acknowledged that his firm does “what we can to get attention” and that “we want to get our piece opened — that’s half the battle.”

I understand what Frost is up against. The Direct Marketing Association reports that letter-sized ad mailings generate an average response rate of about 3 percent. That means a lot of printing and postage dollars are going straight into the trash.

But how far can direct mail advertisers go in the cat-and-mouse game with consumers? The Better Business Bureau in Minnesota says the Midwest Mortgage piece crosses the line. At my request, the bureau reviewed the ad — and found some issues.

“The name of the advertiser should be displayed prominently,” said Dan Hendrickson, the Minnesota bureau’s communications coordinator. “We like people to know who they’re dealing with. With this letter, a person could easily think, ‘Hey, this is a government letter.'”

Hendrickson said the Minnesota office would contact the St. Louis Better Business Bureau and ask the bureau to urge Midwest Mortgage Capital to modify its letter. The firm currently has an A+ rating and no complaints on file with the St. Louis office.

So what makes this letter so tricky? Here’s my take: