emails: fix sending emails to invalid addresses.

Fix mailers to not attempt deliveries to invalid or nonexistent email
addresses. This usually happened when someone changed their email, and
we tried to send a confirmation email to a nonexistent address.

app/models/dmail.rb

@@ -141,7 +141,7 @@ class Dmail < ApplicationRecord
   end
 
   def send_email
-    if is_recipient? && !is_deleted? && to.receive_email_notifications? && to.can_receive_email?
+    if is_recipient? && !is_deleted? && to.receive_email_notifications?
       UserMailer.dmail_notice(self).deliver_later
     end
   end