Exchange does not receive email from Demon - question

[Graham]

MS Exchange on server with nailed up ISDN connection to Demon (i.e. has
fixed IP, is "always on") receives email via SMTP from Demon. This has
worked correctly for 6 months. Nobody has made any changes to Exchange
Server or the Demon email configuration. Demon is still correctly
configured to deliver email using SMTP.

Since yesterday morning, email is not being delivered to the Exchange
server. Internal users can still send emails out from Exchange.

The Demon webmail page shows the waiting mail. Oldest message is curious -
it is shown in black and cannot be viewed. I suspect it may be corrupt. I
know who it is from, so I delete it using the webmail option. Mail
continues to sit there, waiting.

I stop and restart all the services comprising Exchange on the server - no
change.

Finger continues to report messages waiting on punt-3.mail.demon.net

On a computer elsewhere on the internet I set up the mail client to send
messages directly to the SMTP port on the Exchange server - that is, I
replicate what the Demon mail server should be doing. I can successfully
send mail to internal users. In my mind this proves that the Exchange
server is working correctly.

Tried disabling then re-enabling SMTP delivery on the Demon
https://www.password.uk.demon.net/webpassword.cgi - "Mail Options" page.
No improvement.

Can anybody help me further to diagnose the exact problem and correct it?

Thanks.

-- Graham