imap.rb

require 'net/imap'
imap = Net::IMAP.new('imap.gmail.com', 993, true)
imap.login(ENV["GMAIL_USER"], ENV["GMAIL_PASSWORD"])

imap.select("Inbox")
listprefix = "[Imap]/"
listname = "PROCESSED"

unless imap.list(listprefix, listname)
  imap.create("#{listprefix}#{listname}")
end

ids = imap.search(["SINCE", "1-Apr-2000"])
while id = ids.pop
  response = imap.copy(id, "#{listprefix}#{listname}")
  unless response.data.code
    ids.push(id)
    next
  end
  imap.store(id, "+FLAGS", [:Deleted])  
end
imap.expunge

上手くいっている response

#<struct Net::IMAP::TaggedResponse tag="RUBY0009", name="OK", data=#<struct Net::IMAP::ResponseText code=#<struct Net::IMAP::ResponseCode name="COPYUID", data="56 8 13">, text=" (Success)">, raw_data="RUBY0009 OK [COPYUID 56 8 13] (Success)\r\n">

うまくいっていない response

#<struct Net::IMAP::TaggedResponse tag="RUBY0011", name="OK", data=#<struct Net::IMAP::ResponseText code=nil, text="Success">, raw_data="RUBY0011 OK Success\r\n">