I found the bug when the guest book data is saved. 
Autor: SHIMEI N.
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First I saved my guest book data to a file, and next, I changed the saving place from a file to MySQL database.
The result is that the information mail to admin can not come.
So I made another additional guest book which data is saved to MySQL database.
The result was ok, no problem.
Thank you.
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Hello. I sent a notification about your find to the company employees, expect an answer from them here in the comments.
Hello Shimei
I've tested this but it seemed to work fine for me. I believe something else must have been the reason but I am unfortunately unable to reproduce this. Can you reproduce this again by testing with a new Object? If you can, please do let me know
In any case, I will notify the developers of this so that matter remains under control
Thank you
Stefano
Autor
Thank you for answers.