one of the documented procedures in this installation of the ACS
bulkmail_record_failed_host   host
What it does:
Records a host as failed. If host has reached the acceptable failures threshhold, we delete it from the list of hosts.
Defined in: /web/philip/tcl/bulkmail-utils.tcl

Source code:

    ns_share bulkmail_failed_hosts_mutex
    ns_share bulkmail_failed_hosts
    ns_share bulkmail_hosts_mutex
    ns_share bulkmail_hosts
    ns_share bulkmail_current_host

    ns_log Notice "Processing failed host: $host"

    ns_mutex lock $bulkmail_failed_hosts_mutex
    catch {
	set n_failures [ns_set get $bulkmail_failed_hosts $host]
	if { [empty_string_p $n_failures] } {
	    set n_failures 0
	incr n_failures
	ns_set delkey $bulkmail_failed_hosts $host
	ns_set put $bulkmail_failed_hosts $host $n_failures
    ns_mutex unlock $bulkmail_failed_hosts_mutex

    if { $n_failures > [bulkmail_acceptable_host_failures] } {
	ns_mutex lock $bulkmail_hosts_mutex
	catch {
	    set list_index [lsearch -exact $bulkmail_hosts $host]
	    # Check to see if we found this host (the index >= 0)
	    if { $list_index >= 0 } {
		set bulkmail_hosts [lreplace $bulkmail_hosts $list_index $list_index]
	ns_mutex unlock $bulkmail_hosts_mutex
	ns_log Notice "Removed failed host: $host"