Large imap server.

Bill Campbell freebsd at
Tue Feb 14 17:02:13 PST 2006

On Tue, Feb 14, 2006, Francisco Reyes wrote:
>Bill Campbell writes:
>>Cyrus scaling?
>We currently user Courier and so far I am far from impressed with it's 
>We have NFS servers for the storage and then front end machines for the 
>POP/IMAP connections. The footprint per connection is about 3MB to 5MB on 
>the client. On a machine with 2GB that means about 600 connections... but 
>the machines usually are dying around 400 connections.

I'm looking at an ISP's server now with about 7,500 mailboxes with a load
average about 0.29 using courier-imap to serve POP and IMAP clients.  This
server is answering about 4,800 courier connections per hour (mostly POP3).
Granted that POP3 connections are typically very short duration compared to
IMAP which may be open for extended periods of time.

This server is the primary NFS server with 2GB of RAM, with dual 3GhZ
Pentium IVs.  While not a truly low-end box, it probably cost about

This system as one publically accessible MX server that handles all
incoming messages, rejecting about a million connections using a
combination of DNSRBLs and postfix rules.  It runs all the incoming mail
through with postfix, amavisd, and clamav, then passes messages that get
through without virii or phishing to one of four boxes that use
spamassassin to route spam into spam folders.  We chose to run a single
incoming MX server since the spammers would simply cycle amongst backup MX
servers when rejected.

These four boxes run with a load average about 3.0 or less, and have 1GB
RAM on 2.66GhZ Celerons -- not exactly high-end boxes these days.  The
hardware cost of these machines is probably about $600USD each.  Any or all
of these machines could serve POP and IMAP as well, but we prefer to let
them concentrate on spamassassin which is a true RAM hog.

Adding capacity in this type of system is trivial, relatively inexpensive,
and, other than the central NFS user store, not vulnerable to a single
point of failure (each machine has its own openldap server for
authentication which can be rebuilt from the master very easily, which can
be required as the openldap doesn't respond well to unclean shutdowns).

INTERNET:  bill at Celestial.COM   Bill Campbell; Celestial Software LLC
URL:   PO Box 820; 6641 E. Mercer Way
FAX:           (206) 232-9186   Mercer Island, WA 98040-0820; (206) 236-1676

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