What if the postcards don't arrive or are not registered?
They will timeout. When? After 60 days. That should be enough time for a postcard to be sent, travel, and be registered by the receiver.
After that, the user who would have received the postcard will go back to the queue of persons to receive a postcards. If the same user is 12 weeks in the queue, it will be dropped.
My postcard should already been received! What should I do?
Wait

There are several things that can make a postcard take a long time to be registered. Here are a few:
postoffices messing up;
user is on holidays;
user does not have a home internet connection;
user didn't understood your handwritting and is trying to figure out the ID by contacting me;
And no, there isn't anything I can do to help you on that. Please be patiente, old postcrossers will quickly advice you to do that - it works!

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