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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 25 

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.

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Got a packet capture to share? – womble Oct 21 at 5:48
I'm not sure if it matters, but do the clients in question have reverse DNS entries configured? Telnet/ssh and others can be slow if an entry isn't present. Your smtp provider may require it. – Ryan Babchishin Oct 21 at 6:03
Thank you. I don't have a packet capture trace, sorry. How do I capture one? The Reverse DNS theory is something I would like to explore. How do I verify if the reverse DNS entries have been configured? Is this done on a per-client basis? – EmailUser Oct 22 at 5:19

2 Answers 2

The most likely causes of what you have described:

  1. Problems with reverse DNS lookups of your client IP address by the remote server
    IP_Address->[PTR DNS record]->name->[A/AAAA DNS record]->IP_Address

  2. Ident queries timeouts [TCP connections from the remote host to smtp client host]
    Default Ident timeout had used to be 30s, AFAIR modern sendmail uses 5s timeout.

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Thanks. How do I explore the reverse DNS issue further? – EmailUser Oct 22 at 5:19
Use nslookup your_client_ip_address and nslookup received_dns_name. In normal situation you should receive both replies/results below 1s. – Andrzej A. Filip Oct 22 at 5:42
Sorry for the dumb question, but - where do I run the nslookup from? Is it from the email server? The server is a 3rd party hosted site outside our organization. – EmailUser Oct 22 at 6:13
It would be the best location. You can get useful hints running the test from other sites too. – Andrzej A. Filip Oct 22 at 7:01
Here's what nslookup shows: nslookup Server: Address: ** server can't find NXDOMAIN – EmailUser Oct 22 at 7:26

You'll want to respond to revese reverse dns quickly.

One thing you can do with your local dns server (if It's running bind)

$GENERATE 1-254 $ PTR 192-168-0-$

$GENERATE 1-254 192-168-0-$ IN A 192.168.0.$
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