Exchange HTTP redirect method for multitenancy irule

I can't see anything wrong with the logic - here's a idea to move you forward with the Rule though and including some logging;-

when HTTP_REQUEST {

if { [class match [string tolower [HTTP::host]] equals dg_autoDiscover_tenants] && [HTTP::uri] equals "/autodiscover/autodiscover.xml"} {
    log local0. "Redirecting [HTTP::host][HTTP::uri] to [class match -value [string tolower [HTTP::host]] equals dg_autoDiscover_tenants]"
    HTTP::respond 302 Location [class match -value [string tolower [HTTP::host]] equals dg_autoDiscover_tenants] 
return
}  
}

So your datagroup dg_autoDiscover_tenants would have Hosts as keys, and URIs ie https://autodiscover.environment2.com/autodiscover/autodiscover.xml as values. Check for logging output in /var/log/ltm.