My mobile provider (T-Mobile in the UK), used to send spoofed RST packets back to you if you connected to something on port 465, disrupting the TCP connection.

My mobile provider (T-Mobile in the UK), used to send spoofed RST packets back to you if you connected to something on port 465, disrupting the TCP connection. They also used to do that if you connected on port 587, but only if, and not until, you sent "STARTTLS" down the wire. I wrote about it here: https://grepular.com/Punching_through_The_Great_Firewall_of_TMobile https://grepular.com/Punching_through_The_Great_Firewall_of_... You could only send mail if you disabled encryption or used a VPN. They didn't discriminate, they even did this if you were connecting to GMails mail submission service.