Secondray Mail Server

[#1]

Does anyone know if this is possible?
I host a number of domains operate my own mail server which is the primary MX for those domains. The domains come with a single pop3 mail box – a catch all which I use to forward to a plus net mail box so as to act as a secondary mail server. - I use Fetchmail for collect the mail from plus net. I need to do this to cover periods where my ADSL link is down or there is a power failure etc.
What I would like to do is set a plusnet server as the secondary mx and  when say the adsl link comes up after a failure, have the plus net server deliver by smtp. ISTR that there is a way to do this but can't remember how.
Rob

[#2]

http://www.plus.net/support/email/setup/smtp_mail.shtml may help you

[#3]

Can one of the comms team confirm that the information in the link supplied by @jelv is correct. The guide show Other DNS entry being selected and it then show the use of the finger commend. I was under the impression that autoturn servers are only use if SMTP is selected.
@docrob
If you do not want to use PN autoturn servers as these have to be triggered using the finger commend I use these to supply a MX backup.

[#4]

Thanks for the replies.
The problem is that I have a number of domains but they are not hosted by plus.net. I maybe wrong but it looks to me that the setup in the link only works for plusnet hosted domains.
It looks like the solution is to use itsme's suugestion.
Thanks again
Rob