It is almost impossible to exaggerate the importance of mail aliases. Without them, a sendmail system could not act as a central mail server. Mail aliases provide for:
Alternate names (nicknames) for individual users
Forwarding of mail to other hosts
sendmail mail aliases are defined in the aliases file.
 The location of the file is defined in the ALIAS_FILE parameter in the sendmail m4 configuration.
The basic format of entries in the aliases file is:
alias: recipient[, recipient,...]
alias is the name to which the mail is addressed, and recipient is the name to which the mail is delivered. recipient can be a username, the name of another alias, or a full email address containing both a username and a hostname. Including a hostname allows mail to be forwarded to a remote host. Additionally, there can be multiple recipients for a single alias. Mail addressed to that alias is delivered to all of the recipients, thus creating a mailing list.
Aliases that define nicknames for individual users can be used to handle frequently misspelled names. You can also use aliases to deliver mail addressed to special names, such as postmaster or root, to the real users that do those jobs. Aliases can also be used to implement simplified mail addressing, especially when used in conjunction with MX records.
 Chapter 8 discusses MX records.
This aliases file from crab shows all of these uses:
# special names postmaster: clark root: norman # accept email@example.com rebecca.hunt: becky@rodent jessie.mccafferty: jessie@jerboas anthony.resnick: anthony@horseshoe andy.wright: andy@ora # a mailing list admin: kathy, david@rodent, sara@horseshoe, becky@rodent, craig, anna@rodent, jane@rodent, christy@ora owner-admin: admin-request admin-request: craig
The first two aliases are special names. Using these aliases, mail addressed to postmaster is delivered to the local user clark, and mail addressed to root is delivered to norman.
The second set of aliases is in the form of firstname and lastname. The first alias in this group is rebecca.hunt. Mail addressed to rebecca.hunt is forwarded from crab and delivered to becky@rodent. Combine this alias with an MX record that names crab as the mail server for wrotethebook.com, and mail addressed to firstname.lastname@example.org is delivered to email@example.com. This type of addressing scheme allows each user to advertise a consistent mailing address that does not change just because the user's account moves to another host. Additionally, if a remote user knows that this firstname.lastname addressing scheme is used at wrotethebook.com, the remote user can address mail to Rebecca Hunt as firstname.lastname@example.org without knowing her real email address.
The last two aliases are for a mailing list. The alias admin defines the list itself. If mail is sent to admin, a copy of the mail is sent to each of the recipients (kathy, david, sara, becky, craig, anna, jane, and christy). Note that the mailing list continues across multiple lines. A line that starts with a blank or a tab is a continuation line.
The owner-admin alias is a special form used by sendmail. The format of this special alias is owner-listname where listname is the name of a mailing list. The person specified on this alias line is responsible for the list identified by listname. If sendmail has problems delivering mail to any of the recipients in the admin list, an error message is sent to owner-admin. The owner-admin alias points to admin-request as the person responsible for maintaining the mailing list admin. Aliases in the form of listname-request are commonly used for administrative requests, such as subscribing to a list, for manually maintained mailing lists. Notice that we point an alias to another alias, which is perfectly legal. The admin-request alias resolves to craig.
sendmail does not use the aliases file directly. The aliases file must first be processed by the newaliases command. newaliases is equivalent to sendmail with the -bi option, which causes sendmail to build the aliases database. newaliases creates the database files that are used by sendmail when it is searching for aliases. Invoke newaliases after updating the aliases file to make sure that sendmail is able to use the new aliases.
 The AutoRebuildAliases option causes sendmail to automatically rebuild the aliases databaseeven if newaliases is not run. See Appendix E.
In addition to the mail forwarding provided by aliases, sendmail allows individual users to define their own forwarding. The user defines personal forwarding in the .forward file in her home directory. sendmail checks for this file after using the aliases file and before making final delivery to the user. If the .forward file exists, sendmail delivers the mail as directed by that file. For example, say that user kathy has a .forward file in her home directory that contains email@example.com. The mail that sendmail would normally deliver to the local user kathy is forwarded to kathy's account at podunk.edu.
Use the .forward file for temporary forwarding. Modifying aliases and rebuilding the database takes more effort than modifying a .forward file, particularly if the forwarding change will be short-lived. Additionally, the .forward file puts users in charge of their own mail forwarding.
Mail aliases and mail forwarding are handled by the aliases file and the .forward file. Everything else about the sendmail configuration is handled in the sendmail.cf file.