Waving hi from Norway! Long time, no see!
If you go to Help > Contact on the main site and tell the ID of the postcard, the admins will, as far as I know, be able to tell you the username of the person that sent the postcard. Then you can send them a message. @paulo and @meiadeleite, please correct me if I’m wrong!
Last I heard only if it was less than a year expired. I will check
under registering the postcard I got this message “Sorry, but BY-2482231 was sent over 1 year ago and can no longer be registered.”
@lyndac @Norway_girl is correct. Even if the card is more than one year old, contact the admins and tell them the ID number of the received card. If the Postcrosser still has an active account, they will tell you the UserID. You still cannot register the postcard, but you can send a message through the Official site to thank them for the card.
If a postcard has travelled for 365 days or less you’ll be able to register it. But even when it becomes unregistrable, the admins will be able to figure out who sent it, and tell you (as long as the sender still has an account here).
Exactly what @Norway_girl and @ColorfulCourtney have said — just send us a message via the site’s Contact form, and we’ll have a look on the logs for the sender. 
You can still let the person know the postcard arrived, and say thank you!
I have thanked some senders that way, so it worked!
I got a postcard that was sent May 2019 and it has finally arrived (Nov 2020). I want to thank the postcrosser, but there is no way to find their details.
Maybe their account is closed anyway after so many months if they are a new postcrosser who gave up.
Is there anything that can be done to thank the postcrosser and get in contact with them to share the god news that it finally arrived?
In the past I think you could register expired postcards but just they won’t receive a postcard?
Any others thoughts and comments I would be interested to read.
Seaview.
You can’t register it, but you can say thank you. Send a note to the Postcrossing team and they’ll help you.
Thank you @Cassiopheia for letting me know.
I didn’t know I can use the contact button for this.
Thanks for the advice
Seaview
As others have already answered your question, let me just say, what a cute card! I definitely would like to see it on your wall and favorite it. It made me smile 
This afternoon, I was registering the cards that I picked up at the post office. One card though…I got an error message after each attempt to register it. I double & triple checked the ID and everything seemed fine. Then I looked at the message Postcrossing flashed at the top:
Sorry, but RU-XXXXXX was sent over 1 year ago and can no longer be registered.
I am sure this has happened before, but I found this a unique experience.
One of mine came after 181 days…
From Germany, I think.
Oh yesssss, this card is really cute!
I just received a card sent July 2019! It was sooo weird when i got the error message! almost 2 years ago… still it found its way!!
Yes, I agree! Love that card!
I have a question about this, but what is the reason that postcards expire after one year? I understand 60 days to “expire” and send a new one, but I wonder about the 1-year register expiration.
Talking about “expiration time” what do you think of extending the period, as during these Covid times many countries are struggling with mail distribution? Just so the users can have the postcard at their walls. Or at least contact each other for a thank you message without disturbing admin people
I have the same question, I have never understood why postcards “expire.” I get the idea of allowing the sender to send a new card after 60 days, but I do not at all understand why a card cannot be registered when received no matter how long it has been in transit. These are the heroic cards that have survived and incredible journey, they should be celebrated!
Please refer to paulo answer in this link