FAILED selecting pool based on DNS reverse lookup result.

Hi you might be able to use geo-location https://devcentral.f5.com/articles/new-geolocation-capabilities-in-v101.U04lXUp--M8 for what you are trying to do also but I think this is how you use a reverse lookup;-

when CLIENT_ACCEPTED {
    if {!([domain [RESOLV::lookup inet -ptr [IP::client_addr]] 1] eq "jp")} {
        pool SMTP_AUTH_Pool
    else {
        pool SMTP_Fallback_Pool    
    }
}