Yesterday my pen pal showed me that when he got my letter, the top of the envelope was obviously opened with a paper knife, and sealed with transparent tape. 
How could this have happened? Itās an envelope traveled from the UK to France, with nothing but postcards in it!! I canāt think of any reasons but a customs check. But hasnāt X-rays solve that?
Itās really creepy that someone is spying on my private communications. Good thing nothing was lost anyway.
Yes, I wouldnāt assume anything creepy was going on, just likely misdelivered & someone opened it assuming it was for them. Happens a lot.
Thatās a reasonable explanation!! Though judging from the time it was delivered, it is unlikely. But who knowsā¦ Feeling better now.
Happened to me as well! I was confused to find concert tickets in the letter before realizing that it was mail for my neighbor that was misdelivered! I wrote an apology and put it into the correct mailbox. It happens more often than you think!
Do you need a customs form to send non-documents between UK and France? If so, I would guess maybe customs opened it because it felt thicker than a normal letter, and they suspected it contained a gift, like blank postcards or something.
At any rate I would not be too creeped out. If someone really had ill intentions, I think they would have either hidden the evidence by stealing it, or else taken the time to carefully steam it open. The fact that it was delivered in an obviously pre-opened state, seems more like whoever did it felt they had nothing to hide.
I still get that itās uncomfortable to think of anyone - even a customs agent or innocently careless neighbor - reading your personal correspondence, but hopefully they just glanced at it without really reading.
Thatās what I thought too. I sent many thick envelopes to international friends before, and itās just the first time Iāve experienced this, so itās really weird
Yeees thatās exactly what I felt!!
I think, if it were opened by customs, they will seal it again, but not with normal tape.
They use āopened/examined by customsā tape or sticker.
It could have been someone after money? Here, especially during christmas, we are told not to send money in an envelope, because some people try to steal mail, open them, and look what mail has money.
But, it has happened I have opened someones mail too. I have always written a note, that it was misdelivered and I opened it by accident, mostly they were something not personal, but it still feels bad and therefore I write the note, hopefully it makes it feel a little better and not so creepy.
It happened to me but not always. I guess itās the matter of luck or when the envelope is sent by using registered mail (in my case)
Iāve won lottery here it was like more over than 50 cards 
, the big envelope was sent without custom or anything, just plenty of stamps. They didnāt open it and didnāt tax me for those. Iāve also received things like teabags and flat items like stickers and washi tape, the envelope was still sealed and no tax.
In some cases other postcrossers didnāt get this luck, the envelope was opened and they had to pay tax for the small gifts they got
However Iāve received an envelope sent by registered mail and the envelope was only filled by written postcards. I got taxed from it!! (Not much but stillā¦)
I donāt feel creepy about when my envelope was opened, but it makes feel like āam I doing a crime??ā or āis this tax really necessary?? Donāt you have bigger tax evaders to deal with?ā - but welp things can be funny
Thanks everyone for sharing your own experience and thoughts. I googled before posting the topic, and many said it was likely that someone attempted to steal money or anything valuable, which made me really uncomfortable.
Now there are many possibilities a lot better, so I really appreciate your warm reply. 
Anyway letās hope it wonāt happen to any of us!!
Sometimes the mailers donāt properly seal the envelopes they send. I have received envelopes that somehow survived without the contents falling out.
Do I need to pay taxes to send blank cards in envelopes to other countries?
I had a similar experience last December (envelope sent from France to the Netherlands).
The regular envelope contained a total of 6 postcards (5 postcards for a swap + a bonus one).
I checked at the post office for the right amount of stamps and carefully sealed the envelope with extra tape.
After a couple of days I received a thank you message from the receiver with a picture of the received swap cards: she had received all 5 of them but without any envelopeā¦ Luckily I had put the address on all the 5 swap cards and had used decorative sticker stamps on them. The cards were individually postmarked in France, therefore the envelope must have been opened here for a check, sending the cards without envelope to the Netherlands afterwards.
Unfortunately I had put beautiful stamps on the decorated envelope and the 6th (bonus) card did not make it as it was the only card where I did not put the addressā¦ 
I had never sent that many postcards in a regular envelope before, now if I send more than 3 cards I use a big envelope, just in case. I also do not decorate envelopes any longer, keeping them as neutral as possible.
Fyi: last year in August I ordered some French postcards online. When I collected the envelope at the post office, I noticed that it was opened and resealed with some tape. I checked the content and noticed that 10 cards were missing. I contacted the shop and they told me that during the month of August there are temporary post office workers and that things get lost / stolen sometimes unfortunately. They also told me that they had included several extra goodies in the envelope (including a key chain), all of which were missing too.
They kindly offered to give me all the missing items if I would visit their shop in person, which I did later on.
Right, here has also been times when the postal workers stole something. 
It might be good idea to make a complaint/query about the opened letter, so they will perhaps look if there are several cases and to where/whom it can be traced to.
But also, if an envelope is just normal paper, and there is several cards, sometimes the cards are so sturdy, that these rip the envelope. Maybe even fall. Especially when there is something heavier, like a key chain, that can break the envelope and part of contents fall. (Also then itās taped in the post, if they see it, but I think even then they put a sticker noting about it. )
On Taobao or Xianyu,some one seen his postcards were sold.
But after ask the receiver,the answer is not receive yet.
So I think maybe have possibly thing get lost / stolen
On some occations, the customs have opened envelopes sent to me. But then itās clearly marked as opened by customs. Either taped with a tape saying āopened by customsā or a sticker saying so.
I have received one or two envelopes like this before, but Brazilian postal service attached a note explaining that they were damaged during transit to Brazil. Maybe the same thing happened in your case but the postal service didnāt write an explanation. I also had mail opened by Brazilian customs, but in addition to a note, the envelope was sealed again with a special tape.
āDamaged by friction in transitā
It also includes the name of the employee and their ID number.
It was resealed with Brazilian postal service official tape.
Recently, my good friend in East Europe told me he has received an envelope which was opened. Inside were several postcards only and he thinks something was stolen as the sender usually sends him about 10 postcards. He was very disappointed. He is not a Postcrosser but I have read in a Postcrossing profile where the same country as him that in their country, stamps on the postcards can be stolen too.
Such things never really happen in Japan. In Japan, when it is opened by the postal clerk, they add a note to appologize with a reason of opening it.
