A man suing Lindsay Lohan...called her a "Born Druggie"

The stereo installer who is suing Lindsay Lohan is so angry he didn't get paid, he launched a very personal attack, telling her -- "You're a born druggie!"

 Stephen Clark is suing Lindsay for more than $1,100, claiming she stiffed him on an installation in her Venice home while she was on house arrest.

TMZ has obtained correspondence between Lindsay and Clark, which started civilly but quickly deteriorated.

In the first exchange, Clark wrote.  "Hi Lindsay! Well, I sent you two invoices and several BBMs and texts. I have never heard back from you.  So I guess you just aren't going to pay for my labor or your equipment."

Clark continued:  "I am sorry, but I cannot let that much money go."

Lindsay responded, "Sorry!!  I will have it to you this wk!  I'm changing business managers and such so I'm trying to handle it asap."

But time passed, and Lindsay went radio silent.

So a frustrated Clark wrote back, "You're a RECOVERING addict who goes to the santa monica probation dept.  Should I share this as well?"  And Clark continued, "You're a born druggie!"  

Clark is suing Lindsay in small claims court.