@Deepak
I understood that tcp_window_scaling=0 can only limit the speed of traffic - as you linked here - TCP Window Scaling | qa | cafe
But since almost all cloud/dedicated servers comes with 1gbit/sec connection, it’s a questionable how much it affects the speed.
I didn’t tested with lower values of rmem/wmem.
Problem is that this is hardly reproducible problem - sometimes it get stuck with 200 KB email, and sometimes it works flawless even with 18 MB emails.
I don’t know how large emails my customers sent, but since I see timeouts on every few minutes, I feel that even smaller messages get stuck.
But still I believe that larger messages have more chances to get stuck… based on my tests.