Invalid Email Blocking Check-in from Persisting

[AlanLiu96]

With an invalid admin email, checkins that attempt to send an email to the admin account will fail.

Client reported issue with check-ins redirecting to a 500 error page, further investigation showed this error.

Error Log Below:

7380:Started PUT "/up/reservations/check-in/38" for 172.28.252.25 at 2017-04-03 15:16:17 -0400
7381-Processing by ReservationsController#checkin as HTML
7382-  Parameters: {"utf8"=>"✓", "authenticity_token"=>"6OCbXWZMLLRKKzip7VRs8vBpAILWJQ8joWGk2Be6Sx4=", "reservations"=>{"1011"=>{"checkin?"=>"1", "notes"=>""}, "1012"=>{"checkin?"=>"1", "notes"=>""}, "1013"=>{"checkin?"=>"1", "notes"=>""}, "1014"=>{"checkin?"=>"1", "notes"=>""}, "1015"=>{"checkin?"=>"1", "notes"=>""}, "1016"=>{"checkin?"=>"1", "notes"=>""}}, "commit"=>"Check-In Equipment", "user_id"=>"38"}
7383-  Rendered admin_mailer/overdue_checked_in_fine_admin.html.erb (7.3ms)
7384-
7385-Sent mail to undegraduateproduction@yale.edu (53.8ms)
7386-Completed 500 Internal Server Error in 278ms
7387-
7388-Net::SMTPFatalError (550 5.1.1 <undegraduateproduction@yale.edu>: Recipient address rejected: User unknown in virtual alias table
7389-):
7390-  app/models/reservation.rb:257:in `checkin'
7391-  app/controllers/reservations_controller.rb:350:in `block in checkin'
7392-  app/controllers/reservations_controller.rb:341:in `each'
7393-  app/controllers/reservations_controller.rb:341:in `checkin'