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       popa3d - Post Office Protocol (POP3) server


       popa3d [-D] [-V]


       popa3d is a Post Office Protocol version 3 (POP3) server.

       A  POP3  server  operates  on  local  mailboxes on behalf of its remote
       users.  Users can connect at any time to check their mailbox and  fetch
       the  mail  that has accumulated.  The advantage of this "pull" approach
       is that any user with a simple POP3-capable  mail  reader  program  can
       receive mail, eschewing the need for a full-fledged Mail Transfer Agent
       (MTA) and a permanent network connection.

       Note that POP3 can only be used to retrieve mail, not to send  it.   To
       send mail, the SMTP protocol is commonly used.

       For  access  to  a  mailbox  through  POP3, the username must be in the
       password database.  Additionally, popa3d does not permit null passwords
       and will refuse to serve mail for root (UID 0) users.


       -D     Standalone  server  mode.   In  this  mode, popa3d will become a
              daemon, accepting connections on the  pop3  port  (110/tcp)  and
              forking child processes to handle them.  This has lower overhead
              than starting popa3d from  an  inetd  equivalent  (which  popa3d
              assumes by default) and is thus useful on busy servers to reduce
              load.  In this mode popa3d also  does  quite  a  few  checks  to
              significantly reduce the impact of connection flood attacks.

       -V     Print version information and exit.


       A  normal  POP3 session progresses through three states: AUTHORIZATION,

       After the TCP connection opens, the client must authenticate itself  to
       the  server during the AUTHORIZATION state.  The following commands are
       supported in the AUTHORIZATION  state  (all  command  names  are  case-

       USER name
              Authenticate as user name.

       PASS string
              Authenticate using password string.

       QUIT   Quit; do not enter UPDATE state.

       When  authorization  is  successful,  the server enters the TRANSACTION
       state.  The client can now list and retrieve messages or mark  messages
       for  deletion.  The following commands are supported in the TRANSACTION

       DELE msg
              Mark message for deletion.

       LAST   Show highest message number accessed (obsolete).

       LIST [msg]
              List message number and size.

       NOOP   Do nothing.

       QUIT   Quit; enter UPDATE state.

       RETR msg
              Retrieve message.

       RSET   Clear deletion marks.

       STAT   Return total number of messages and total size.

       TOP msg n
              Show top n lines of message body.

       UIDL [msg]
              List message number and digest.

       When the client issues the QUIT command in the TRANSACTION  state,  the
       server  enters  the  UPDATE  state.   All messages that were marked for
       deletion are now removed.  The server then closes the connection.


       POP3 transmits passwords in plaintext and thus, if you care  about  the
       security  of  your individual user accounts, should only be used either
       in trusted networks or tunneled over encrypted channels.

       There exist extensions to the protocol that are supposed  to  fix  this
       problem.   popa3d  does not support them yet, partly because this isn’t
       going to fully fix the problem.  In fact, APOP and the  weaker  defined
       SASL  mechanisms  such  as CRAM-MD5 may potentially be even less secure
       than transmission of plaintext passwords  because  of  the  requirement
       that plaintext equivalents be stored on the server.


       Solar Designer <solar at>

       This manual page is based heavily on the one Camiel Dobbelaar wrote for


       Official Internet Protocol Standard STD 53, also known as RFC 1939.