mark_read.rb

#!/usr/bin/ruby
require 'net/imap'

server = 'imap.mail.com'
username = 'YOUR USERNAME'
password = 'YOUR PASSWORD'
folder = 'INBOX'

imap = Net::IMAP.new(server, 993, true)
imap.login(user, password)
imap.select(folder)

imap.search(["NOT", "SEEN"]).each do |message_id|
  imap.store(message_id, "+FLAGS", [:Seen])
end

imap.logout()
imap.disconnect()