Internet Express for Tru64 UNIX Version 6.8 Installation Guide (14234)

can enable only one IMAP server on the system at any given time. You can switch from one
server to another at any time. See the Administration Guide for more information.
This section compares and contrasts the UW-IMAP server with the Cyrus server by considering
several factors (mail file format, user setup, administrative overhead, mail folder sharing, and
disk quota management). HP recommends the Cyrus IMAP server for its superior performance
and scalability features, especially at sites having a large number of users who might store a
large volume of mail. Cyrus also provides automated disk quota management, which is a benefit
to Internet service providers (ISPs), who need to control the disk space consumed by users' mail.
The Cyrus server is also included in Tru64 UNIX.
2.19.1 Mail File Format
The UW-IMAP server relies on the standard UNIX mbox mail folders to hold the mail it serves.
The messages in each folder are stored in a single file and are separated using a From header.
This is the format that Sendmail uses to store messages in the /var/spool/mail directory.
Mail-reading programs like dtmail and mailx also use this format. Because it relies on a
standard mailbox format, the UW-IMAP server provides the following benefits:
• Users can easily access messages in existing mbox mail folders without assistance from a
system administrator.
• Legacy mail agents can share mail folders. (But you must ensure that IMAP clients and mail
readers do not manipulate mail folders directly at the same time; otherwise, the folders
might be corrupted.)
• Users can directly access mail folders from the command line.
• The user's INBOX is /var/spool/mail/username.
The mbox folder format has the following drawbacks:
• Performance decreases significantly when folders get large, because all folder operations
require rewriting the folder when the folder is closed.
• The UW-IMAP server writes a temporary copy of open folders to the /tmp directory,
requiring this directory to be sufficiently large. This use of the /tmp directory can also limit
the number of simultaneous connections.
• With the UW-IMAP server, multilevel folder hierarchies are possible only when using UNIX
directories that contain other directories or mail folder files. Some IMAP clients cannot create
directory hierarchies; you must manually create them on the server using the mkdir
command. This limitation can also cause some mail clients to display warning messages if
they attempt to treat these directories as real mail folders.
The Cyrus server uses a private mailbox format. Mail is stored in a spool area that is not accessible
by users. Each mail folder is stored in a directory with caching files, and each mail message is
stored as an individual file in one of the folder directories. The Cyrus mailbox format provides
the following benefits:
• Excellent performance of all folder operations.
• Requests are handled with lower I/O overhead. Note, however, that the one file per message
storage scheme tends to result in a large number of small files, which may require file system
tuning.
2.19.2 User Setup
With the UW-IMAP server, mail is normally stored in a subdirectory of the user's home directory
(usually named ./mail). Many IMAP mail clients allow the user to specify which directory to
use as the mail directory. Some mail clients can to correctly determine which directories are mail
folders, even when the mail directory is not specified. Other clients list all files and directories,
including those that are not mail folders.
2.19 Choosing an IMAP Server 31