Provided by: popa3d_1.0.3-1build1_amd64 bug


       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, TRANSACTION, and

       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-insensitive).

       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 state.

       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 OpenBSD.


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