pop.gmail.com is an alias, which your ISP dns might have messed up trying to resolve.
you could set outlook to use: gmail-pop.l.google.com
which is the real "A" record (one of many) for pop.gmail.com
note that's a letter L in there not a one
you don't want to hard-code your own dns entry for pop.gmail.com because, it can change - and you wouldn't know.
this way if the ip changes, it'll still resolve correctly