FIX: retry sending an email in 1 hour when SMTP server is busy

app/jobs/base.rb

@@ -173,10 +173,9 @@ module Jobs
 
       if exceptions.length > 0
         exceptions.each do |exception_hash|
-          Discourse.handle_job_exception(exception_hash[:ex],
-                error_context(opts, exception_hash[:code], exception_hash[:other]))
+          Discourse.handle_job_exception(exception_hash[:ex], error_context(opts, exception_hash[:code], exception_hash[:other]))
         end
-        raise HandledExceptionWrapper.new exceptions[0][:ex]
+        raise HandledExceptionWrapper.new(exceptions[0][:ex])
       end
 
       nil

app/jobs/regular/user_email.rb

@@ -150,6 +150,22 @@ module Jobs
       [message, nil]
     end
 
+    sidekiq_retry_in do |count, exception|
+      # retry in an hour when SMTP server is busy
+      # or use default sidekiq retry formula
+      case exception.wrapped
+      when Net::SMTPServerBusy
+        1.hour + (rand(30) * (count + 1))
+      else
+        Jobs::UserEmail.seconds_to_delay(count)
+      end
+    end
+
+    # extracted from sidekiq
+    def self.seconds_to_delay(count)
+      (count ** 4) + 15 + (rand(30) * (count + 1))
+    end
+
     private
 
     def skip_message(reason)

lib/email/sender.rb

@@ -135,8 +135,8 @@ module Email
 
       # Suppress images from short emails
       if SiteSetting.strip_images_from_short_emails &&
-         @message.html_part.body.to_s.bytesize <= SiteSetting.short_email_length &&
-         @message.html_part.body =~ /<img[^>]+>/
+        @message.html_part.body.to_s.bytesize <= SiteSetting.short_email_length &&
+        @message.html_part.body =~ /<img[^>]+>/
         style = Email::Styles.new(@message.html_part.body.to_s)
         @message.html_part.body = style.strip_avatars_and_emojis
       end