Can I send mail from p's to iD Le ?

My cousin had an iD Le two weeks ago. Today I'm in my cousin's house. But, my cousin was at school. I brought my p's for connect to her iD Le. I heard that P's can send mail to others. Can p's send mail to iD Le ?

Yes and No, but in your context, No.

After having connected to play games together, sometimes when your Tamagotchi iD L character goes out into their backyard, they'll receive a letter from a previous connection friend. These are random events and their messages are autogenerated from several form-letters. e.g. "It was nice to see you. Come play with me again." Once these letters are read (which happens in the backyard, next to the mailbox) they disappear. ("letter" downloads on the iD L get stored in the treasurechest and can be read and re-read as you wish.)