All you need to do is have the code continue on error, then post to the other server and read the status from the post. Something like this:
PostURL = homelink & "CustID.aspx?SearchFlag=PO"
set xmlhttp = CreateObject("MSXML2.ServerXMLHTTP.3.0")
on error resume next
xmlhttp.open "POST", PostURL, false
xmlhttp.send ""
status = xmlhttp.status
if err.number <> 0 or status <> 200 then
if status = 404 then
Response.Write "ERROR: Page does not exist (404).<BR><BR>"
elseif status >= 401 and status < 402 then
Response.Write "ERROR: Access denied (401).<BR><BR>"
elseif status >= 500 and status <= 600 then
Response.Write "ERROR: 500 Internal Server Error on remote site.<BR><BR>"
else
Response.write "ERROR: Server is down or does not exist.<BR><BR>"
end if
else
'Response.Write "Server is up and URL is available.<BR><BR>"
getcustomXML = xmlhttp.responseText
end if
set xmlhttp = nothing