SMTP email delivery

[#1]

Sorry if this has been answered before, but I can't find the exact answer I need.
I'm dabbling with a trial of SBS 2008 & want to use it (temporarily) as my email server for one of my domains.
I understand about changing the email settings to SMTP & changing domain settings from POP3 to specify DNS records, but the question is, do I really have to set up an A & MX records to point to my static IP or should it do it automatically?
I have tried to configure the aforementioned settings, but no email came through. Am I trying it too quickly? What's a reasonable time to wait before things are configured?
These are the settings I have tried:
mail.domain.co.uk.    a          12.345.678.9
mail.domain.co.uk.  mx  3    mail.domain.co.uk.
                                      mx  20  mx.last.plus.net.

I did have it working fine before with SBS 2003, but no joy this time
Cheers

[#2]

The DNS Servers take up to 24h to up date.

[#3]

Selecting SMTP will automatically change the MX records, no need to do it manually. Just do a MX lookup to see when the changes ripple through.

[#4]

Hmm, set it up over 24 hours ago but an mx lookup still points to 212.159.9.1 which I believe is PN's own.
Anything to do with it being the weekend perhaps?

[#5]

No it has nothing to do with it being the weekend, it is all done automatically, as long as your settings are wright.

[#6]

Quote from: conival

Hmm, set it up over 24 hours ago but an mx lookup still points to 212.159.9.1 which I believe is PN's own. Anything to do with it being the weekend perhaps?

As Gary has mentioned, the systems are entirely automated so this won't be anything to do with the weekend.
All you really needed to do was select SMTP delivery on our website and then enter the IP address for delivery. It looks like you chose the hard option though and tried setting the records up yourself.
A dig against your domain returns this at the moment:

;; QUESTION SECTION:
;domain.tld.         IN      MX
;; ANSWER SECTION:
domain.tld.  14400   IN      MX      20 mx.last.plus.net.
;; ADDITIONAL SECTION:
mx.last.plus.net.       28276   IN      A       212.159.9.1
mx.last.plus.net.       28276   IN      A       212.159.8.1

I reckon it's the host name you've put on the left hand side of the MX record that's confusing things. I've made some changes at this end so DNS propagation allowing, things should now be sorted.

[#7]

Thanks Bob, that might explain matters a little.
I did try it the easy way first, but nothing changed (even allowing 24 hours) so I then tried the manual setup around 16:00 hours today. After 4 hours things sprang to life, MX records were fine, but the PN domain settings had all cleared (obviously by yourself) to default..
Just don't understand why the automatic/ simple method didn't work first time.
Anyway, no worries, it was just something I was experimenting with. It'll all get reverted back to POP in a day or two.
Thanks for all your input & assistance