AutoMapperExtensions.cs

    public static class AutoMapperExtensions
    {
        public static IMappingExpression<TSource, TDestination>
          IgnoreAllNonExisting<TSource, TDestination>(this IMappingExpression<TSource, TDestination> expression)
        {
            var sourceType = typeof(TSource);
            var destinationType = typeof(TDestination);
            var existingMaps = Mapper.GetAllTypeMaps().First(x => x.SourceType.Equals(sourceType) && x.DestinationType.Equals(destinationType));
            foreach (var property in existingMaps.GetUnmappedPropertyNames())
            {
                expression.ForMember(property, opt => opt.Ignore());
            }
            return expression;
        }
    }

    // USAGE: 

    Mapper.CreateMap<SourceType, DestinationType>()
                .ForMember(prop => x.Property, opt => opt.MapFrom(src => src.OtherProperty))
                .IgnoreAllNonExisting();