Help needed to interpret "Delivery Status Notification (Failure)" message

Hello Experts

Could someone please explain to me exactly what the following "bounced" email message means:

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The following message to <deliberately_hidden@sbcglobal.net> was undeliverable.
The reason for the problem:
5.4.7 - Delivery expired (message too old) [Default] 451-'4.7.7 Excessive userid unknowns from 212.74.100.52'
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From the "ATTxxxx.DAT" file returned with the failed email as an attachment:

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Reporting-MTA: dns; mk-filter-1-a-1.mail.uk.tiscali.com

Final-Recipient: rfc822;deliberately_hidden@sbcglobal.net
Action: failed
Status: 5.0.0 (permanent failure)
Remote-MTA: dns; [207.115.36.20]
Diagnostic-Code: smtp; 5.4.7 - Delivery expired (message too old) [Default] 451-'4.7.7 Excessive userid unknowns from 212.74.100.52' (delivery attempts: 76)
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My ISP is http://www.tiscali.co.uk and 212.74.100.52 is one of their IP Addresses (mk-filter-1-a-1.mail.uk.tiscali.com).

The intended recipient's ISP is sbcglobal.net which uses their old "Prodigy" servers.  IP Address 207.115.36.20 is one of theirs (nb-mx-vip1.prodigy.net).

My failed emails have all been coming back indicating that sbcglobal is rejecting those that have passed through the Tiscali servers using IP Addresses 212.74.100.52 through to 212.74.100.55.

It would seem to me to be a Blacklisting issue.  The IP Address 212.74.100.51 (mk-filter-4-a-2.mail.uk.tiscali.com) seems to be immune to this, because I have successfully sent email to the same recipient by logging into my account in Webmail interface.

Webmail messages seem always to be sent through 212.74.100.51, whereas Outlook Express messages are always rejected showing IP Addresses 212.74.100.52 to 212.74.100.55, but never 212.74.100.51.

I had the same issue (coincidentally?) exactly a year ago and, after liaising with idiotic 1st Level Tiscali Support "technicians" via their online help, it was eventually bumped up to another department and the issue was resolved/resolved itself within a day.

This time I believe I have the same idiotic 1st Level Support brain-donors who either misread each of my increasingly annoyed responses without referring to previous information given, or suggest "checking to make sure the recipient does not have you in the blocked sender's list".  

The first response from them stated:
"I understand that you are experiencing issue sending email from email client with a Tiscali email account.  Please note we have limited support for the email client Outlook Express. To investigate the cause, I request you to access the email account from the webmail interface."

What?!!!   "Limited support for the email client Outlook Express".
I don't want to log into Webmail and be bombarded with Flash adverts, nor to have to keep refreshing pages, and I want permanent offline retrievable emails.  Why should I have to use Webmail when I have a pop3 email account?  This notion was, of course, quickly dispensed with in my response.

What I would like to know is sufficient information about the possible reasons for that failure error occurring so that I can demand that something be done (if I am in the position to demand dependent on your explanations).

It's the "Excessive userid unknowns from xxx.xx.xxx.xx" part that I need to know about.

What "UserID"?
Does this indicate that my ISP is not applying compliant headers, or that the sbcglobal spam filtering software is being overzealous?

Could it infer that sbcglobal's servers have PREVIOUSLY been bombarded with spam messages with mismatching information in the headers from (or purporting to be from) Tiscali IP Addresses, and that the spam filters have blocked the IP range I mentioned?

I really need a well informed suggestion as to the most likely cause for the failure message.

What isn't helping is that it is taking 3 DAYS to get the "Delivery Status Notification (Failure)" messages returned to me, so it's hard to respond to the so-called Support staff I have been liaising with and trying to convince that the problem is not with my computer or the recipient's (I've checked all the settings using VNC connection).

It's hardly surprising that the reason for eventual return of my email became "5.4.7 - Delivery expired (message too old)", given that my ISP bombarded the sbcglobal mail servers 76 times over 3 days until conceding that they wouldn't accept the message.

Could this bombarding actually start or perpetuate a blacklisting due to the fact that each attempt is perceived as a new email?

Your experienced views on this would be helpful.

Thanks
Bill