UNIX System Administration Handbook - Evi Nemeth [325]
The aliasing scheme we use lets the alias files be the same on all machines within an administrative domain. This uniformity is a big win from the administrator’s point of view. (We assume that login names and UIDs are unique across all machines, a policy we recommend highly.)
Some sites centralize mail by exporting /var/spool/mail over NFS. Locking problems with NFS files can cause users to lose mail or to have their spool files garbled. Finger pointing among NFS, sendmail, and the delivery agents with regard to proper locking does not help the poor user whose mailbox has been corrupted (however, sendmail is always innocent, since it never actually delivers mail).
Some NFS implementations (such as those on dedicated NFS file servers) include a lock manager that works correctly. Most implementations either do not support locking or support it improperly. Some sites just ignore the locking problem and hope for the best, and others require users to read mail on the file server.
Our best advice is just not to use an NFS-shared /var/spool/mail.
Using IMAP or POP
IMAP and POP are protocols that download email to a user’s desktop machine when it joins the network. It is the ideal way to manage mail, especially for hosts that are not always connected, either because they are turned off when not in use or because they are at home and share the phone line with teenagers.
IMAP, the Internet Message Access Protocol from the University of Washington, is our favorite of these tools. It delivers your mail one message at a time rather that all at once, which is much kinder to the network (especially on a slow link) and better for someone traveling from location to location. It is especially nice for dealing with the giant Microsoft attachments that some folks like to send: you can browse the headers of your mail messages and not download the attachments until you are ready to deal with them. IMAP manages mail folders among multiple sites, for example, between your mail server and your PC. Mail that stays on the UNIX server can be part of the normal backup schedule. www.imap.org contains lots of information about IMAP and a list of available implementations.
POP, the Post Office Protocol, is similar but assumes a model in which all the mail is downloaded from the server to the PC. It can be either deleted from the server (in which case it might not be backed up) or saved on the server (in which case your mail spool file grows larger and larger). The “whole mailbox at a time” paradigm is hard on the network and less flexible for the user. It can be really slow on dial-up lines if you are a pack rat and have a large mail spool file. Mail ends up getting scattered around with POP. A reasonable implementation of POP is available from Qualcomm at www.eudora.com/qpopper.
The IMAP server software is available from www.washington.edu/imap. No configuration is needed except to put the proper IMAP entries in the /etc/services and /etc/inetd.conf files and to make sure that your firewall (if any) doesn’t prevent it from working. IMAP has been guilty of security problems in the past; see the CERT advisories and be sure to get the latest version.
19.4 MAIL ALIASES
Aliases allow mail to be rerouted either by the system administrator or by individual users.6
They can define mailing lists, forward mail among machines, or allow users to be referred to by more than one name. Alias processing is recursive, so it’s legal for an alias to point to other destinations that are themselves aliases.
sendmail supports several aliasing mechanisms: LDAP (the Lightweight Directory Access Protocol), NIS and NIS+ from Sun, NetInfo from NeXT/Apple, mail routing databases, and various alias files that users and system administrators can set up.
If you want to use the mail homes concept, we recommend that you implement it by storing your aliases in an LDAP server. The alternatives have several disadvantages. Flat alias files usually must be maintained by root. Their database representation must be rebuilt