mailaddr —
mail addressing description
Mail addresses are based on the Internet protocol listed at the end of this
  manual page. These addresses are in the general format
user@domain
where a domain is a hierarchical dot separated list of subdomains.
    For example, a valid address is:
eric@CS.Berkeley.EDU
Unlike some other (now obsolete) forms of addressing, domains do
    not imply any routing, or the existence of a particular host. Simply because
    mail may be sent to ``user@somedomain.com'' does not imply that there is any
    actual host named ``somedomain.com'', and does not imply a particular
    routing of the message. Routing is performed by Mail Transport Agents, such
    as postfix(1), based on
    policies set in the MTA's configuration.
Under certain circumstances it may not be necessary to type the entire domain
  name. In general, anything following the first dot may be omitted if it is the
  same as the domain from which you are sending the message. For example, a user
  on ``calder.berkeley.edu'' could send to ``eric@CS'' without adding the
  ``berkeley.edu'' since it is the same on both sending and receiving hosts.
  Whether abbreviation is permitted depends on how your site is configured.
Domain names (i.e., anything after the ``@'' sign) may be given in any mixture
  of upper and lower case. Most hosts accept any combination of case in user
  names, although there are exceptions.
Every site is required to have a user or user alias designated ``postmaster'' to
  which problems with the mail system may be addressed, for example:
postmaster@CS.Berkeley.EDU
Certain old address formats, such as UUCP ``bang path'' addresses, explicitly
  routed internet addresses (so-called ``route-addrs'' and the ``percent hack'')
  and others have been used historically. All these addressing formats are now
  considered obsolete, and should no longer be used.
To some extent, MTAs attempt to provide backward compatibility for
    these addressing forms, but in practice many of them no longer work. Users
    should always use standard Internet style addresses.
mail(1)
D. H. Crocker,
    Standard for the Format of Arpa Internet Text
    Messages, RFC, 822,
    August 1982.
mailaddr appeared in 4.2BSD.
The RFC 822 group syntax (``group:user1,user2,user3;'') is not supported except
  in the special case of ``group:;'' because of a conflict with old
  berknet-style addresses, not that anyone cares about either berknet or group
  syntax style addresses any longer.