Sending warning letters with respect to infringement of patents is a common exercise common, yet the practice with respect to formulation of such letters differs widely within the EU.
In Austria, the practice in sending warning letters to (alleged) infringers is rather liberal. If there is a granted patent, the patentee can excessively refer to the instruments the patent entitles the patentee (eg. threat of preliminary injunction or of imprisonment). This practice is only limited by §1 Act of Unfair Competition (AUC; preventing acts contrary to good morals in commercial activities), for example if the patentee warns despite knowledge of invalidity of the patent or if the patentee has acquired the patent in bad faith.
The situation is different, if the addressee is not the actual infringer, but another firm, for example a customer of the infringer. This has been analysed in a recent decision of the Austrian Supreme Court (4Ob184/06x) wherein the patentee has sent a warning letter to a customer of the plaintiff informing this customer that the plaintiff infringes their patents. The plaintiff sued the patentee to stop the filing of such warning letters to their customers.
The Austrian Supreme Court made a clear distinction between the warning letters sent to the infringer and a warning letter sent to a customer of this infringer. Whereas the direct warning to the infringer falls under §1 AUC (addressee has to prove that this is an “act contrary to good morals”), sending of the warning letter to a customer of the alleged infringer falls under §7 AUC (“Whoever, [..] alleges or disseminates facts concerning the enterprise of another, concerning [..], which facts are apt to harm the operation of the enterprise or the credit of the proprietor, shall be liable to pay damages to the injured party, if such facts cannot proven to be true. [..]”). In this case, the sender of the warning letter carries the burden of proof. Accordingly, if the patentee is in the position to prove the infringement, also the sending of a warning letter to customers of the infringer is allowed.