Exchange SMTP relay probs

[MK-2]

I've hardly had any experience with Exchange so please bear with me if this is something ridiculously easy.
I've got on to the exchange server (was meant to be an external company supports it but i can't get through to them and its getting more uregency as time passes). when users are sending emails, they are getting a reply any time up to a day later saying the message has been delayed, do not resend etc. but it never makes it out.
I looked in queues and under the smtp relay all the emails are sat there waiting on RETRY.
looked under connectors and smtp relay and it has forward all email to smtpmail.adslmail.co.uk
i've been told the school possibly has its bband via pipex (but no confirmation yet). ive tried changing the smtp relay to smtp.dsl.pipex.com in case, but still no luck. incoming is fine, but all outgoing emails are now sitting at exchange saying it cant get out.
is there anything i should be looking for in particular?
it just doesn't look good that the problem started 1 day after i started. and i haven't changed anything on the server so as far as im aware it isn't me that's changed anything to cause this

**edit**
ok since typing this out, the messaves have moved in the queue from the smtp relay to one labelled 'messages with an unreachable destination'
i know its not the external addresses as one is my home address that sent a successful incoming email.
also in the monitoring and status section, it lists smtp relay and smtp dns.
if i try to right click on either, i get a message about a snap in for mmc has failed do i want to restart mmc (doing so fixes nothing). and under status, dns is ok but smtp relay is unavailable.

[MK-2]

very weird. just went back into exchange admin and it had sent them all. crisis over (for now) but i dont know what had caused it!

[SYNACK]

Edit: Oops, was still writing my response when you posted yours, this could still be useful for troubleshooting any future problems though:

Are you able to use port 25 directly through your ISP, if you are you could set the connector to send the messages directly from your exchange server using DNS.

You can test the connections to your upstream providers by telneting in with command prompt and making sure that you can connect

telnet smtpmail.adslmail.co.uk 25

If it does not connect then you need to find out who is blocking it, your ISP or the mail provider.

You can use the same method to check to see if you can send it directly by attempting to telnet to the mail server that handles your private emails. If you can connect you should be able to send all mail using DNS directly.

This setup can involve a bit of extra work though as some email providers (Yahoo ) will see you mail server as a spam sender and junk all incoming emails from you unless you ask them nicely to allow your ip address through.

[MK-2]

heh, ive been here 4 days. i dont know who the ISP is, i dont know any external IP addresses!
but if its working for now i'll leave it. its obviously connecting at some stage to send them. if it starts playing up again i'll address the problem. otherwise i'll mark it as (somehow) fixed