Alternate names (nicknames) for individual users
Forwarding of mail to other hosts
 The location of the file is defined in the "Options" section of the sendmail configuration file.
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.  This aliases file from almond shows all of these uses:
 Chapter 8, Configuring DNS Name Service , discusses MX records.
# special names postmaster: clark root: norman # accept firstname.lastname@example.org rebecca.hunt: becky@peanut jessie.mccafferty: jessie@walnut anthony.resnick: anthony@pecan andy.wright: andy@filbert # a mailing list admin: kathy, david@peanut, sara@pecan, becky@peanut, craig, anna@peanut, jane@peanut, christy@filbert 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 almond and delivered to becky@peanut. Combine this alias with an MX record that names almond as the mail server for nuts.com, and mail addressed to email@example.com is delivered to firstname.lastname@example.org. 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 nuts.com, he can address mail to Rebecca Hunt as email@example.com 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. 
 If the D option is used (see Appendix E, A sendmail Reference), sendmail automatically rebuilds the aliases database - even if newaliases is not run.
In addition to the mail forwarding provided by aliases, sendmail allows individual users to define their own forwarding. The user defines her 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 firstname.lastname@example.org. 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 the user in charge of his own mail forwarding.