Quite frequently, clients send me copies of official-looking documents that they have received relating to a non-U.S. patent, trademark or domain name. The documents typically describe an upcoming deadline and request that the client pay a fee in order to keep the patent, trademark or domain name in effect.
My advice to clients who receive these letters is generally this: “if you didn’t receive it from your attorney, it’s probably a scam.”
My colleague Christopher Kinkade recently published a useful article describing the USPTO’s official warning about such scams for trademarks, along with a companion article relating to domain name registration scams. To read Chris’ articles, click here (trademarks) or here (for domain names).
IP Spotlight 
There is one notable exception to the rule “if you didn’t receive it from your attorney, it’s probably a scam” and that is the notice of loss of rights issued by the EPO when a PCT application designating Europe fails to enter the regional phase by the 31 month deadline.
In that case the notice of loss of rights is sent direct to the applicant (because of course, no relevant representative is known to the EPO at that stage)
@Sean: great comment. Perhaps a better rule is “if you don’t receive it from your attorney or a government authority, it’s probably a scam.”