Lawyers send cease-and-desist letters, not as a courtesy, but because of cntinued action after notice is served, which results in a substantially higher liability.
Cease-and-desist letters will NEVER say, "Stop (whatever you're doing) or we'll sue you." A threat to sue, or even identifying conduct as infringing, is usually enough to grant subject matter jurisdiction for a declaratory relief action, i.e. the recipient of such a letter can go to court as a plaintiff and sue the sender, seeking that the judge issue a judgment declaring the plaintiff not guilty.
Accordingly, cease-and-desist letters will always be phrased along the lines of, "Should you fail to comply with the demands herein, [CLIENT NAME] may avail itself of such rights and remedies provided pursuant to applicable state and federal law, and said rights and remedies are expressly reserved herein."
The strategic use of the word, "may." Should anyone ever be on the receiving end of such a letter, it is perhaps noteworthy that just because it doesn't say, "Stop or we'll sue you" doesn't mean that you're not about to get sued.
However, it would be a rarity. It is simply not cost-effective (even for large corporations) to do so. It depends on the content of the email, though. You may like to seek the advice of a lawyer on this one if you feel that its a cause for concern.