You could either create your own login page that will forward their details to the real login page. Other than that you could install your own webmail application on a new sub-domain, currently I’m using roundcube which looks great but does have some issues as it’s still an early beta release.
I personally dislike the Webmails that are provided on mail.*.com etc. Like Padwah, I also use RoundCube which looks and works great. There are a couple of bugs, but they’ve recently opened a new forum to deal with things quicker etc.
It’s not a problem in this instance, as the customer just wants to put their logo as well as terms and conditions and a welter of other legaleze mumbo-jumbo on the sign in page…
Don’t know what OS you use, since it relates, I’ll post it here…
There’s a component for ASP.Net that allows you to perform POP3 functions by Craig Pickles.
It’s not a complete client solution like Cube or Squirrel, but if your clients want to customize their apps, they can write an entire back and front end for this with their own branding and layout.
I wrote a rough framework for testing and it seems fairly stable.