If you send in your delete or nomail command properly it will be carried
out regardless of conceal, if you have the right address.  Some nodes
send error reports which have different addresses for the users than
are in your subscription list.  These places sometimes report errors
for the strictly local form of the address while you have the "outside
world" form of the address.  Can take some sleuthing to match them up.
You didn't send a copy of the error so can't say more on that, except
there is one place in Australia that does this which has driven me crazy
at times.  Write to the postmaster.
 
A handy way to find concealed users is to send in q listname for *@*
and then use an editor to search for Conceal=Yes.
 
        Douglas Winship    Austin, Texas    [log in to unmask]
                    Secondary AUTOCAT Listowner