Slow performance with large folders over the Internet

[Gerard Ranke]

On 03/31/2017 12:03 AM, Shawn Heisey wrote:
> Dovecot package version is 1:1.2.15-7+deb6u1.  It is in Debian 6.0.10,
> using the Debian package.
> 
> The server is in my basement at home, and is exposed to the Internet so
> I can fully access my mail from anywhere.  I use IMAP for reading mail.
> 
> I have a number of folders in my mailbox that have thousands of messages
> in them, from mailing lists.
> 
> When I'm at home, I have a LAN connection to the server.  It goes
> through a Cisco firewall that limits the connection speed to 100Mb/s.
> In this situation, I can open a folder with 25000 messages in it, click
> on the next unread message that Thunderbird did not know about before,
> and within a second or two, the message will download, allowing me to
> view it and reply.
> 
> When I'm at work, with highly variable network latency between
> Thunderbird and the server, doing exactly the same thing takes a LOT
> longer.  I have seen it take as long as 15 minutes for a single message.
>  If I open a folder with only a few messages in it, it is fast.
> 
> The server is not overloaded -- I can log into it with ssh and use "mutt
> -f" to open a folder directly.  Loading thousands of messages into mutt
> takes a while, but I have no difficulty using the ssh connection and
> running commandline programs.
> 
> This suggests that the IMAP communication between the server and the
> client involves a large amount of back and forth communication when the
> message count in the folder is high, possibly something for every
> message in the folder.  It happens quickly on a LAN but crawls on a
> connection with high latency.  I can understand it taking a few seconds
> longer on a high-latency link, but it takes minutes.
> 
> I do plan on building a new server and migrating to Dovecot 2.x, but I
> haven't had the time to work on that.
> 
> Is this a known problem? If so, is it fixed in 2.x?
> 
> Thanks,
> Shawn
> 

Hi Shawn,

If you think that imap is the problem, you can do an imap session by
hand and see where the problems are:

openssl s_client -CApath /path/to/your/certs -connect your.server:143
-starttls imap

See fi. http://wiki.linuxquestions.org/wiki/Testing_IMAP_via_telnet

But from your mail I would say that you might have networking or
firewall issues. So I would be looking for interface errors, missing
ping packets, traceroute output and so on.
Best,

gerard