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The MercuryI IMAP4rev1 server 90
About IMAP

The MercuryI IMAP4rev1 server

About IMAP

IMAP is an Internet protocol that allows you to access mail folders on a remote computer sys-
tem. There are several versions of IMAP, but the most widely-used version these days is
called IMAP version 4, or IMAP4. Mercury only supports the IMAP4rev1 variant of the pro-
tocol and will not work with clients requiring older versions: in practice, this is unlikely ever
to be an issue.

System requirements

Most Internet protocols are reasonably "light-weight", but the nature of the IMAP protocol
makes it much more demanding, especially of memory. We recommend that you calculate
the memory requirements for the machine where MercuryI will be running as 500KB per user
connected at the same time. So, if you have 10 users connected to MercuryI, it will be using
approximately 5MB of virtual memory. This calculation is necessarily very rough - if your
users have few folders then it will be substantially less, and if your users have many folders
(more than 1000) then it may be substantially more. MercuryI allows simultaneous connec-
tions to the same mailbox: when simultaneous connections exist to the same mailbox, only
the first will typically incur any memory overhead - the other connections are essentially
"free". During normal operation, MercuryI may consume significant amounts of disk space
in the Windows temporary directory, so make sure that plenty (at least 100MB) is always
available.

Client configuration
At present, MercuryI presents the Pegasus Mail message store, which does not support the
idea of folders that can contain both messages and other folders: folders can contain either
messages, or other folders, but not both. You may need to configure your IMAP4 client to
take account of this fact - for instance, in Pegasus Mail, when you create an IMAP profile for
a MercuryI server, you would make sure that the "This server supports folders within folders"
control is not checked. Future versions of MercuryI and Pegasus Mail will almost certainly
allow folders to contain both messages and other folders.

Configuration

Unlike the POP3 protocol, which allows users to access their new mail only, the IMAP pro-
tocol allows users to access all their folders on the server, and to see them presented in the
familiar hierarchical layout. MercuryI is the Mercury protocol module that provides IMAP4
access to your users' compatible IMAP-compliant mail clients, such as Pegasus Mail, Eudora,
Mulberry and Microsoft Outlook. The version of the IMAP protocol supported by MercuryI
is called IMAP4rev1, and it is documented in Internet Standards document RFC3501; clients
specifically designed for use with earlier versions of the IMAP protocol may or may not work
correctly with MercuryI, but at the time of writing, practically all widely-used IMAP clients
were known to be compatible.

IMAP is probably the single most complex protocol of all the protocols in regular use on the
Internet – it is substantially more complex than the SMTP protocol used to send mail, the
HTTP protocol used to access web pages, or the POP3 protocol used to service new mail fold-
ers. Given this complexity, it is paradoxical that MercuryI is probably the easiest of the Mer-