Author: Daniel Mettler

If you often encounter error messages like

“Server your_imap.server.org has disconnected. The server may have gone down or there may be a network problem.”

when trying to access Courier-IMAP from an IMAP client (e.g. Mozilla Thunderbird), there are two approaches to solve the problem:

(I) “The real fix”

By default, Courier-IMAP only allows 4 concurrent connections per IP address for clients accessing it. Usually, this isn’t sufficient for modern IMAP clients which cache several connections and hence try to keep several connections opened in parallel. For example, the popular (and recommendable) Mozilla Thunderbird mail client tries to cache 5 IMAP connections by default, exceeding the number of connections allowed by Courier-IMAP by one. This causes several problems, apart from the one mentioned above, Thunderbird may be slow or have trouble to successfully display new e-mail messages at all (i.e. it only displays a white page instead of the content). Now, fixing this isn’t difficult, as long as you have root access to your IMAP server:

1. Edit the configuration file of Courier-IMAP (in my case that’s /etc/courier-imap/imapd) and increase the value of the MAXPERIP setting. For example, I increased MAXPERIP from 4 to 20 (which works fine for me so far):

##NAME: MAXPERIP:0
#
#  Maximum number of connections to accept from the same IP address

MAXPERIP=20

Note that you do not need to add MAXPERIP to the Courier-IMAP-SSL configuration file (/etc/courier-imap/imapd-ssl on my box) as options in the imapd-ssl configuration file augment the options in the imapd configuration file.

2. Then restart your Courier-IMAP server(s). If you run Gentoo like me, just execute

# /etc/init.d/courier-imapd-ssl restart
(and ‘/etc/init.d/courier-imapd restart’ if you also run a non-encrypted IMAP server)

(II) “The workaround fix”

The above “real fix” (I call it the “real” one as Courier-IMAP’s default setting of MAXPERIP=4 is too restrictive nowadays indeed) is only applicable if you have administrator rights on your IMAP server. If you don’t have, you may either want to contact your server’s administrator or decrease the number of concurrently cached IMAP connection in your IMAP client. E.g. for Mozilla Thunderbird, do the following:

1. Open “Tools -> Account Settings…”
2. Select the “Server Settings” of your IMAP account
3. Open the advanced settings dialog by clicking on the “Advanced…” button
4. Lower the “Maximum number of server connections to cache” from 5 (default) to e.g. 2

(For other IMAP mail clients, change the settings accordingly)

BTW If you use Microsoft Outlook, consider setting your mail options according to the following Outlook quoting recommendations (PDF, 225 KB) or rather switch to a better mail client (e.g. Mozilla Thunderbird, Evolution, KMail, .. almost anything but Outlook will do it ;)

See also the following post by Omar Shahine (Architect of MS Entourage‘s IMAP support) on IMAP, Thunderbird, and mail clients: Entourage earns an A-, Outlook a D+, Outlook Express a C, Windows Eudora a F and Mozilla Thunderbird a B+ :) His executive summary:

“Thunderbird is an almost perfect IMAP client for Windows. If you use IMAP, this is the product for you.”

In any case, have fun! :)