Is "keep up the good work" a polite thing to say to your co-workers? I'm under impression that only someone from higher position has the privilege to say this.

And what are other alternatives for this kind of "good job!" praising?

Is "keep up the good work" a polite thing to say to your co-workers? I'm under impression that only someone from higher position has the privilege to say this.

When replying the emails to a co-worker, who is not a subordinate of yours, is "keep up the good work" a proper phrase in this case?

What other praises I could use to express my sincere respect on what they have achieved, instead of a perfunctory "good job!"?