I've been asked several times recently whether a debt collector can serve someone with a lawsuit by mail. In Minnesota, the answer is yes, but only if you sign a form acknowledging receipt of the lawsuit. Merely mailing you a lawsuit is not valid service. If you don't sign and return the acknowledgment, there is no service and you are under no obligation to respond to the lawsuit. Apparently, some debt collectors are mailing lawsuits to consumers as a collection tactic. I imagine the belief is that the consumer will believe they have been served with the lawsuit and, out of fear, immediately call up the debt collector to make payment. If that is, in fact, the intent, I believe the approach is misleading. And depending on how the debt collector's cover letter is worded, this tactic may be a violation of the Fair Debt Collection Practices Act (FDCPA).
If you receive a debt collection lawsuit in the mail, only sign and return the acknowledgment if you intend to answer the lawsuit within 20 days of the date you sign the acknowledgment. If you sign and return the acknowledgment, but don't answer the lawsuit, a default judgment will probably be entered against you.