Let us look in RealDomain and then in the RealDomain:-solve command. One is addressed to the usual solve command. The commands of the RealDomain package are not still documented since Maple 7 when the package was introduced. There is a general description only 

• By default, Maple performs computations under the assumption that the underlying number system is the complex field. The RealDomain package provides an environment in which computations are performed under the assumption that the basic underlying number system is the field of real numbers.
• Results returned by procedures are postprocessed by discarding values containing any detectable non-real answers or replacing them with undefined where appropriate.

The above is not enough. Here is an example which confuses me: 

RealDomain:-solve(exp(I*x) = -1, AllSolutions);
NULL

though 

solve(exp(I*x) = -1, AllSolutions);
                         Pi (2 _Z1 + 1)

and 

RealDomain:-solve(exp(I*x) = -1);
                               Pi

I lie awake thinking about that. Maplesoft staff help me!