Could someone explain the following Ruby on Rails error?

LoadError in CandidatesController#create

Unable to autoload constant Usermailer, expected Z:/railsassignment/student/app/mailers/usermailer.rb to define it

When I submit a form I get the error above. The form processes a record and the candidate is added to the database however the welcome email I'm trying to send to the newly registered candidate doesn't send, and the error above prevents the user from proceeding.

Candidates Controller

def create
  @candidate = Candidate.new(candidate_params)

 respond_to do |format|
   if @candidate.save
     Usermailer.welcome(@candidate).deliver_now ***<-- Error highlights this line***
     format.html { redirect_to @candidate, notice: 'User was successfully    
   created.' }
     format.json { render :show, status: :created, location: @candidate }
    else
     format.html { render :new }
     format.json { render json: @candidate.errors, status:   
 :unprocessable_entity }
   end
 end
end

usermailer.rb

Z:/railsassignment/student/app/mailers/usermailer.rb (usermailer directory)

class UserMailer < ActionMailer::Base
 default from: "from@example.com"

def welcome(candidate)
 @candidate = candidate
 mail(:to => candidate.can_email, :subject => "Welcome to EmployeMe.com, You    
 have registered successfully!")
 end
end

Should you need to see any more of the files drop me a comment and I'll be quick to add them to the question.