OK, I've a fairly complex mail delivery path - in the general case. It works something like this:

smtp.mydomain.com -- Standard single host on a static IP address. This has a catch-all email address - this is necessary, though I do understand that it is not currently recommended.

smtp.mydomain.com uses fetchmail to fetch mail from:
myaccount1[at]lineone.net
myaccount2[at]lineone.net
myaccoutn3[at]shareddomain.org
myaccount4[at]myisp.com

mydomain.com has three MX servers defined:
0. smtp.mydomain.com
1. mx0.123-reg.co.uk
2. mx1.123-reg.co.uk

When smtp.mydomain.com is unavailable the 123-reg MX server and 123-reg backup MX server kick in... these support mail-forwarding... and I've configured various forwards based upon username (for email addresses of the form username[at]mydomain.com) - which, subsequently sends email to my 4 pop accounts (to ensure the right bits are accessible by the right people - even if the primary mail-server smtp.mydomain.com is down.)

shareddomain.org has two MX servers
lineone.net has six MX servers - as far as I can tell - mx1.uk.tiscali.com to mx6.uk.tiscali.com.
myisp.com has a single MX server.

I hope that is at least reasonably clear... if complicated. :-)

I've got 6 Mailhost name/email address entries listed unser Mailhosts. These are:

1. mydomain.com - 2 domains - 1 IP address
2 myisp.com - 11 domains - 6 IP addresses
3 [MX0] myaccoutn3[at]shareddomain.org - 2 domains - 1 IP address
4 [MX1] myaccoutn3[at]shareddomain.org - 2 domains - 1 IP address
5 myaccount1[at]lineone.net 46 domains - 30 IP addresses
6 "mike" mydomain.com [using backup MX @ 123reg.co.uk] - 29 domains -20 IP addresses

So, my questions...

A) Does it matter that I've lots of candidate domains/IP addresses for some of the entries? Does it matter which one I select, or are all the domains/IP addresses in the drop-down box now assumed OK?
Is there a straightforward way to verify that I've caught all the entries I need to supply?
C) Is there any problem that I'm going to receive mail at thousands of different usernames of the form username[at]mydomain.com? They should all be processed as I outlined above.

Thanks in advance...

Steve

A lot of this has already been addressed in my attempted FAQ as MailHost Configuration problems ....

The only "sure" way to verify .... just as stated in the FAQs, the Pinned statements, so many other Previous Post, Topics, and Discussions .... submit a spam to the parser, check the results ....

I admit "C" is fairly clearly explained... (Sorry)

Your clarification on "B" was helpful (Thanks)

While I suspect that the answer to "A" might be documented somewhere, by any chance, would you mind either confirming or giving me a pointer to the correct FAQ answer?

If, for example, tiscali.co.uk were to change their mailserver configuration, would I need to re-configure my mailhosts in SpamCop, or will this be automatically detected from the DNS?

Have you looked at FAQ Entry: MailHost Configuration problems? Essentially you need to nominate an address for each possible host. You do need to keep an eye out for changing configurations. It happens, though not very often in such a way to affect the actual hosting - and even then may not affect you every time anyway; if, for instance, tiscali is used by a number of reporters then the first one of them updating their hosts for tiscali will update the others (tiscali users) too. IIUC.

Getting more than a bit peeved about a number of things .... if you read that FAQ, then you'd have noted the lack of support from the paid-staff. There was a recent thread in the spamcop newsgroup that answered this very question .... was going to point you to the newsgroup archives, only to find that Don has once again set his newsreader to "X-No-Archive: yes" again ... which means his posts won't get archived ....

I got around that problem by quoting his post, such that it will in fact get archived ..... of course, what's avtually needed is to tranfer that data to that attempted FAQ .... you know, the one for which there are no FAQ type answers ...... geeeze ......

Anyway, I just checked and the quote has been archived .. please see [Scspamcop] MyRealBox .. actually, see that post, then read the next one, which is me quoting Don's answer ....