Q:

How to deal with a lying customer? 

I'm a freelancer and deal mostly with web, a client of mine has hired me to deal with a monthly newsletter: improve the deliverability and the message itself, I signed up for a popular online service (don't know if I can say the name) made a template for them to use with it, imported the mailing list they had and showed them how to use the service, deliveries and opens skyrocketed and the cost lowered over 50%, the client was so happy with the results we signed a contract with a monthly fee to manage this service and do other jobs related to web development and consulting. 
All fine and good until this one guy who works at this company, I believe he was the one who hired the previous service, began complaining about not receiving the mails.  So I logged into the management software and there it was - the guy had reported the message as SPAM, the first time I removed his email from the spam list - resend and when I asked him if the message were getting through he told me it went into the spam folder, ok... the second time , again he complained about the message, but this time he opened the message just to send it to spam again, I hate to assume the worst of people, but now I'm sure he's doing it on purpose, because the software uses the images in the email to register when a user opens the message. 
It's obvious that he's lying, but I don't want to create a conflict inside the client - he appears to be a good accountant , what is the best way to gracefully deal with such a situation. 

A:

Whenever you run across an issue like this, you frame it like an accident. " Well, it looks like you accidentally opened the email and then flagged it as spam.  I know you probably go through a lot of emails every day and it's easy to accidentally miscategorize something, but you might try to pay extra care not to flag this newsletter". 
You can also go to his supervisor/manager/whatever, same concept: "I just wanted to let you know that I looked into the issue that Joe reported, and it looks like he accidentally flagged the email as spam a couple of times." 
This way you directly address the problem without placing any blame of intention.  If he's doing it on purpose, he'll probably stop now that attention has been drawn to it.  If it was honestly an accident, he'll probably pay more attention.