It isn't often that I receive a letter that leaves me angry, fuming, and then yelling over the phone at a representative of a business I've been a customer of for years. But it just happened and reminds me that it's always a good time to review what works in marketing and what only seems to work. Because when marketing backfires, it can break the critical relationships with customers that keep you in business.

Emotional resonance is the heart of marketing because selling is an emotional act. You're speaking to someone's heart. But will you be a lover or a seducer? A lover cares for the other person and understands the importance of the relationship. The seducer just wants to get into someone's ... wallet.

Being a patient of a medical care provider is an example of an ongoing relationship. The patient depends on the provider to be honest, straightforward, and helpful. The care provider should, in response, care about the same things as well as ethics and duties of proper practice. These are relationships that typically last for years.

For many decades, professional organizations of all sorts insisted that their members refrain from advertising. That has changed over time and we all realize businesses need to reach out. For a medical practice, it is not only necessary to maintain and grow revenue, as is true for any other business, but imperative to help patients get the proper care, whether those people are generally inclined to or not.

The two impulses can crash in ugly ways. The letter I received from an ophthalmology practice I've used for at least a decade was certified, typically a sign of some serious communication.

A brief paragraph didn't address a critical test result or even insurance payment gone awry. Instead, it said that they wanted to schedule an appointment with one of the doctors and that I needed to be in touch for the care "necessary to treat your condition." One last sentence asked that I call to schedule an appointment "in the near future."

Condition? Other than wearing glasses, that was news to me. And near future? That isn't the language you use for something that needs to be addressed immediately.

I called and was told that an annual exam was a good idea and that the condition was diabetes. Except, I'm not diabetic. Oh, but an eye doctor can sometimes see signs of an onset of diabetes, I was assured.

The entire letter's approach was a marketing come-on. I blew up on the phone and told them I would now have to consider whether I could continue to see the practice. A patient has to trust a care provider and they had just abused that trust.

The letter had attempted to exploit fear, long known as one of the most reliable motivators in marketing, especially direct response. Sometimes the fear can be used legitimately. For example, your car could use an annual inspection to be sure everything is working correctly, which is why many states mandate such examinations. Or it can be used to make someone worry when unnecessary.

Had the practice said something to the effect that an annual eye exam was useful and could uncover early signs of diabetes, things would have been fine. You notify the patient -- the customer -- to deliver important information. But the reference to someone's "condition" crossed a line, playing to the specific fear that something serious might be wrong. Sending the letter via certified mail only heightened the result.

Although the office manager claimed that it was just a general letter they were sending out, the structure and delivery were clear, particularly as this practice is good at self-promotion. It's unlikely someone would have come upon the combination of language and additional cost of certified mail by mistake.

By all means, use emotional connections with customers and prospects. But recognize that there are limits to what you should do in your hunger for financial success. As yourself whether you're trying to maintain a long-term relationship with people who keep your company afloat or sell vacation time-shares to swamp-surrounded properties in Florida. Better to have a sustainable business than to embrace the art of the con man.