I'm using a Linux client machine, connecting to a 3rd party external mail server.
The mail server takes extremely long to respond when I try sending mail. To debug this, I installed the mailx basic client and tracked the communication between client and server. After narrowing down further, it turns out that the telnet to port 25 is the slow step, even when I provide the IP address of the server (i.e. no DNS lookup is involved).
Thus, I can reproduce the slowness by running the following command at the Linux command line prompt:
% telnet 220.127.116.11 25 Trying 18.104.22.168...
Waits for several seconds before either aborting or going ahead.
The strange part is - this slowness is only seen on some client PCs - with Xubuntu 14.04.
% uname -a Linux myPC 3.13.0-24-generic #46-Ubuntu SMP Thu Apr 10 19:11:08 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
The slowness isn't seen on a Fedora system. It's also not seen on the same system when I connect to the same mail server through a different ISP. That's probably because the first ISP is an office network, where many users are simultaneously trying to connect their mail clients to the same mail server.
Any hints would be appreciated.