A thought: your post as well as the existing answers seem to take for granted that you must read the letter before handing it over. I don't see where you explicitly said that. Assuming your concern is that the possibly-untrustworthy people might be involved in something dangerous and/or illegal, you could instead deliver the letter as promised and then retrieve it afterward. To make sure you'll be able to pick it out of a pile of other papers, you might "accidentally" drop it in some unusually colored ink so that a corner gets thoroughly soaked. Then you or an ally can break into the home/office/doomsday bunker of the recipient and seek out and read the note with the big spot on it. Suggested spells for this approach: knock, invisibility, dark vision, and any other thiefy spell you can get your hands on.
If your your concern is for the recipient--that, trustworthy or not, your service may be used as a vehicle to do them harm--you could always just tell the recipient you sensed magic, you're worried the sender might be malicious, and ask them if this magic is expected. If this freaks them out you could ask permission to magically open the envelope at a safe distance. Even if they don't go for this, their reaction is likely to tell you something. Suggested spells: mage hand, a look of sincere concern.
You can also try to get the recipient to let you check the letter out, then break in if that fails.
Semi-serious fallback plan: if the letter turns out to have information you want to go away, burn the place down and hope the recipient doesn't remember the finer details. Suggested spells: grease, firebolt
This answer doesn't give you an opportunity to withhold or delay the message, but that's covered by the other answers, and I feel like it has an unacceptable risk of backfiring. Would you send a message important enough to involve magic via Random Schlub Courier Service without some plan to verify delivery?