| I remember having to deal with the early GFW about 20 years ago when I was working for a company that had some employees on a site in Shanghai. Every morning, our colleagues in China would open their mail client and it would connect to our server abroad. The first person would usually be OK, but for everyone else, the connection would fail. At the time, almost nothing was known of the GFW and it wasn't as clever as it is now. I found out that the POP connection was quickly blocked after a few minutes, probably triggering some slow firewall rules along the way (it seemed a bit random, so I assumed the firewall setup wasn't unified). Moving to POPS/SMTPS seemed to improve things for a while, but the connection would still be randomly blocked. What worked in the end was to use a bunch of random ports instead of the well known ones to accept POP/SMTP connections on the server, and we never had any issues after that, at least until we changed system a couple of years later. |