The storyline of "you have got mail" and "kaadahl Kottai" are very similar, though Kadhal kottai was the earlier movie. How did this happen?

Long story.

There is this 1950's movie called Shop around the corner.
A telegu author copied this story into a serial in a magazine.
Director R.Balu lifted this story and started to make this movie with Thangar Bachchan as a the cameraman. The movie was stalled for some reason. Thangar lifted the story and told it to Ahathiyan. When the movie was released, Balu threatened legal action. Shivasakthi Pandian,the producer of Kadhal Kottai, gave him another movie as a settlement. the movie was Kaalamellam Kadhal Vaazhga.

Shop around the corner was remade as "You have got mail" with the snailmail being replaced with email.

Long live originality!!