[exim] limit concurrent pipes

    Here is our configuration. We have one alias with more then 1000 
aliased addresses. When we sending mail to this alias we are in error 
"to many files open", as ulimit for Linux is 1024 open files. Is this 
possible to limit concurrent pipes? Something like max_rcpts option?

ldap_aliases:
    driver = redirect
    allow_fail
    allow_defer
    one_time = true
    data = ${lookup ldap{LDAP_AUTH \
        LDAP_ROOT_ALIASES?mailAliasedName?sub?(&\
        (mailAlias=${quote_ldap:$local_part}))}}
    file_transport = address_file
    pipe_transport = address_pipe

ldapuser_local:
    driver = accept
    condition = ${lookup ldapdn{LDAP_AUTH \
        LDAP_ROOT_ACCOUNTS??sub?(&(accountStatus=TRUE)\
        (mail=${quote_ldap:$local_part}${quote_ldap:@}\
        ${quote_ldap:$domain}))}{yes}{no}}
    transport = ldap_delivery

ldap_delivery:
    driver = pipe
    command = /usr/libexec/dovecot/fixdeliver -d $local_part@$domain
    message_prefix =
    message_suffix =
    delivery_date_add
    envelope_to_add
    return_path_add
    log_output
    user = mail
    environment = UMASK=0007 : UNAME=$local_part : DOMAIN=$domain

With respect,
    Boris