expn - recursively expand mail aliases
expn [-a] [-v] [-w] [-d] [-1] user[@hostname] [user[@hostname]]...
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.
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
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
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 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.
No environment variables are used.
/tmp/expn$$ temporary file used as input to nslookup.
aliases(5), sendmail(8), nslookup(8), RFC 823, and RFC 1123.
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
Lastly, expn does not handle every valid address. If you have an
example, please submit a bug report.
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
The latest version of expn is available through anonymous ftp at
David Muir Sharnoff <email@example.com>