All of my reports to spam.spamcop.net fail because whilst there's an MX record for spam.spamcop.net, there's no A record so sendmail says
<submit.xxxxxxxxxxxxx[at]spam.spamcop.net>... Unrecognized host name spam.spamcop.net.

This started happening a few weeks ago. Is there a mailertable hack I can do to get around it? I'm using sendmail 8.14.2.

I don't know what your system is doing...it should not be looking for an A record for spam.spamcop.net. It should be finding the MX record for spam.spamcop.net and using vmx1.spamcop.net or vmx2.spamcop.net

Perhaps you installed some kind of update about the time the problem started happening? Have you checked the lookup manually that you did not place some kind of local hosts table?

Microsoft Windows [Version 6.0.6001]
Copyright © 2006 Microsoft Corporation. All rights reserved.

C:\Users\sunderwood>nslookup
Default Server: resolver1.opendns.com
Address: 208.67.222.222

> set type=mx
> spam.spamcop.net
Server: resolver1.opendns.com
Address: 208.67.222.222

Non-authoritative answer:
spam.spamcop.net MX preference = 5, mail exchanger = vmx2.spamcop.net
spam.spamcop.net MX preference = 5, mail exchanger = vmx1.spamcop.net
> set type=any
> vmx2.spamcop.net
Server: resolver1.opendns.com
Address: 208.67.222.222

Non-authoritative answer:
vmx2.spamcop.net internet address = 204.15.82.29
>

It is indeed likely to be a sendmail issue not a spamcop issue, hence the subject "sendmail woes".

[njh[at]bandsman ~]$ ping spam.spamcop.net
ping: unknown host spam.spamcop.net
[njh[at]bandsman ~]$

[root[at]bandsman log]# fgrep spam.spamcop.net maillog | tail -1
Jun 12 16:20:02 bandsman sendmail[15049]: m5CFK2t2015049: m5CFK2t3015049: DSN: <
submit.xxxxxxxxxxxxx[at]spam.spamcop.net>... Unrecognized host name spam.spamcop
.net.
[root[at]bandsman log]#

This is NOT a sendmail forum... you will need to do some of your own work here. I am trying to give you hints that might help in your search.

The ping is not relevant. There is no host spam.spamcop.net.

Your system seems to think that 3 level domains can not have their own mail server and instead interprets it as a hostname in the spamcop.net domain. In Lotus Notes (my last job), there were connection documents to define this (i.e. *.* = internet, *.*.* = internet, etc.)

Any mail server is supposed to be doing an MX record lookup on the right side of the address first before attempting an A record lookup. Figure out why it is not, and your problem is solved.