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EXPN(1) General Commands Manual EXPN(1)


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 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.


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 = 0.

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 a line)


The latest version of expn is available through anonymous ftp at


David Muir Sharnoff    <>

March 11, 1993 7th Edition