Problem with W5100 ethernet shield

Hi everyone.
I recently developed a sketch that allows to send mail with Arduino Uno and W5100 ethernet shield.
It uses a simple SMTP server without TLS cipher, and is based on the EMailSender library from Renzo Mischianti, that i reduced to adapt them to my basic board (the original library was too big)

Now it works good, but i think there's some kind of problem with ethernet shield.
I prepared a simple test sketch that send a text mail with 30 secs of delay.
It send exactly 6 mail and after the TCP messages from the server appears to be truncated. So i need to soft-reset the board and it restart working proprerly for other 6 email.

Seems to be a buffer overflow in w5100.

Anyone knows more about this kind of problem?
Thank you in advance.

See if Bug #2 applies : The W5100 bug(s) – and how to fix it (them) – Arduino, ESP8266, ESP32 & Raspberry Pi stuff