def delete_all # :yield: message
      mails().each do |m|
        yield m if block_given?
        m.delete unless m.deleted?
      end
    end
Deletes all messages on the server.

If called with a block, yields each message in turn before deleting it.

=== Example

n = 1
pop.delete_all do |m|
File.open("inbox/#{n}") do |f|
f.write m.pop
end
n += 1
end

This method raises a POPError if an error occurs.