require 'net/imap'

HOST = ''
USER = ''
PASSWORD = ''

imap = Net::IMAP.new(HOST, 993, true)
imap.authenticate('LOGIN', USER, PASSWORD)
imap.select('INBOX')
imap.search(['UNSEEN']).each do |message_id|
  puts message_id
  imap.store(message_id, '+FLAGS', [:Seen])
end
imap.close
imap.logout