For Pop3 class, the default TLS/SSL mode is SslMode.None. If you want to use TLS 1.2, please specify SslMode explicitly like this:
client.Connect("testtls.pec.rupar.puglia.it", 995, SslMode.Implicit)
Since the port 995 is default for implicit SSL/TLS, you can also use this call:
client.Connect("testtls.pec.rupar.puglia.it", SslMode.Implicit)