A lambda expression with statement body cannot be converted to an expression tree

            return _employeeRepository.Get().SingleOrDefault(employee =>
            {
                var mailPrefix = employee.LoginName.Split('@')[0];

                if (!string.IsNullOrEmpty(employee.LoginName)
                    && employee.IsActive.Value 
                    && !string.IsNullOrEmpty(employee.AdGuid)
                    && string.Equals(mailPrefix, guestLoginName))
                {
                    return true;
                }
                return false;
            });

Error: A lambda expression with statement body cannot be converted to an expression tree

Separating your predicate logic for the employee to standalone bool method should work - a simple modification of the code:

private bool IsEmployeeLoginNameValid(Employee employee)
{
   var mailPrefix = employee.LoginName.Split('@')[0];

                if (!string.IsNullOrEmpty(employee.LoginName)
                    && employee.IsActive.Value 
                    && !string.IsNullOrEmpty(employee.AdGuid)
                    && string.Equals(mailPrefix, guestLoginName))
                {
                    return true;
                }
                return false;
}

Then just use it like this:

return _employeeRepository.Get().SingleOrDefault(IsEmployeeLoginNameValid);

Or

return _employeeRepository.Get().SingleOrDefault(employee => IsEmployeeLoginNameValid(employee));

EDIT: If you .Get() return IQueryable, you will have to materialize the collection first in order to make my suggestion work - otherwise it could throw an error -> I guess you are using EF and standard queries via repository translates the LINQ to native SQL query. So any local method/expression will not work because the query provider will not be able to transform it to sql query.