NAME

       expn - recursively expand mail aliases

SYNOPSIS

       expn [-a] [-v] [-w] [-d] [-1] user[@hostname] [user[@hostname]]...

DESCRIPTION

       expn  will  use  the  SMTP  expn  and  vrfy  commands  to expand mail aliases.  It will first look up the
       addresses you provide on the command line.  If those expand into addresses  on  other  systems,  it  will
       connect  to  the  other  systems and expand again.  It will keep doing this until no further expansion is
       possible.

OPTIONS

       The default output of expn can contain many lines which are not valid  email  addresses.   With  the  -aa
       flag,  only expansions that result in legal addresses are used.  Since many mailing lists have an illegal
       address or two, the single -a, address, flag specifies that a few illegal addresses can be mixed into the
       results.   More -a flags vary the ratio.  Read the source to track down the formula.  With the -a option,
       you should be able to construct a new mailing list out of an existing one.

       If you wish to limit the number of levels deep that expn will recurse as it traces addresses, use the  -1
       option.   For  each -1 another level will be traversed.  So, -111 will traverse no more than three levels
       deep.

       The normal mode of operation for expn is to do all of its work silently.  The following options  make  it
       more  verbose.   It  is  not necessary to make it verbose to see what it is doing because as it works, it
       changes its argv[0] variable to reflect its current activity.  To see how it is expanding things, the -v,
       verbose,  flag  will  cause  expn to show each address before and after translation as it works.  The -w,
       watch, flag will cause expn to show you its conversations with the mail daemons.  Finally, the -d, debug,
       flag will expose many of the inner workings so that it is possible to eliminate bugs.

ENVIRONMENT

       No environment variables are used.

FILES

       /tmp/expn$$ temporary file used as input to nslookup.

SEE ALSO

       aliases(5), sendmail(8), nslookup(8), RFC 823, and RFC 1123.

BUGS

       Not all mail daemons will implement expn or vrfy.  It is not possible to verify addresses that are served
       by such daemons.
       When attempting to connect to a system to verify an address, expn only tries one IP address.   Most  mail
       daemons will try harder.
       It  is  assumed that you are running domain names and that the nslookup(8) program is available.  If not,
       expn will not be able to verify many addresses.  It will also pause for a long time unless you change the
       code where it says $have_nslookup = 1 to read $have_nslookup = 0.
       Lastly, expn does not handle every valid address.  If you have an example, please submit a bug report.

CREDITS

       In  1986  or so, Jon Broome wrote a program of the same name that did about the same thing.  It has since
       suffered bit rot and Jon Broome has dropped off the face of the earth!  (Jon, if you are out there,  drop
       me a line)

AVAILABILITY

       The  latest  version  of  expn  is  available  through  anonymous  ftp  at  ftp://ftp.idiom.com/pub/muir-
       programs/expn.

AUTHOR

       David Muir Sharnoff    <muir@idiom.com>