According to iiNet's support web page,

iiNet? ;-)

Oops, sorry, I'd just been dealing with iiNet support :-)

According to Westnet's Global Roaming page
http://www.westnet.com.au/internet/additional/global.aspx

Mac might need to be using the iPass Dial Wizard.
The iPass Dial Wizard appears to be for dial-up access from locations
other than WA or Aus?

iPass is for outside Australia -- you can use your Westnet dial-up Australia wide, through their single telephone number.

Isn't Mac trying to gain access from Internet Cafes? Won't these
generally be on broadband?
If he is going through a cafe's broadband shouldn't he be using
mail.westnet.com.au?

Each different ISP will have a different SMTP server associated with it. So with each location that Malcolm drives the Motor Home to in Canada, he'll most likely need to use a few different SMTP servers.

Westnet seems to be saying that mailr.westnet.com.au is only for use
through the iPass software:

Oh yes, a good point, you're correct. It does appear to work for normal senders of Mail too though. YMMV.

I am on WN broadband and mailr.westnet.com.au doesn't respond to any
pings, only mail.westnet.com.au
does.

Don't bother trying to ping common addresses, or to use this as a "gauge" of whether an address is alive or not. You're wasting your time, as it's not a true representation as to whether the server is alive or not. Most ISPs turn off ping responses from their common servers (from example DNS servers, many Mail servers etc.). I asked iiNet about this practice a few years ago, and they gave a few responses. They said they either don't like the extra traffic (= performance hit), or their firewall blocks the pings (helps reduce attacks if people can't see your server easily), or they don't like to advertise their server is "available" to use as a "test" to see if it's alive. All stands to good reason.

So, pinging is a less than reliable measure these days. Perhaps instead, you could try telnetting to the server on say port 25, and sending a few raw SMTP commands instead. (There are web pages dedicated to this). Or, do a DNS lookup (local or web-based), which might provide some info as to it's "alive" status.

I cant see why Westnet themselves haven't sorted this out for Mac.
One long-distance call to support should fix this I reckon.

The problem doesn't affect Westnet customers in general, it affects ALL mobile users. Malcolm has already been on the right track, which he described as grabbing the "domain" part of local email address of the service provider his particular location has been using. While not full-proof, he's on the right path. It's just some ISP's might have an email address of "[EMAIL PROTECTED]", and their SMTP address might be "smtp.superisp.com.au", and others might use "mail.superisp.com.au". It will vary each time Malcolm changes ISP or location. :-)

Dare I ask; what is wrong with using Westnet's webmail to access your
emails while abroad?

Malcolm could use Westnet's webmail; this avoids the SMTP changing routine. BUT I'm sure it's down to the convenience of being able to use Malcolm's default Mail application, without having to make too many "changes" to his routine, access to his address book etc.

When I'm on the road, I much prefer to use my default Mail app where possible, so that I have easy access and tracking to previous threads of messages etc. :-). But if my laptop is not convenient, or if I can't work out the SMTP address of a particular service provider, or similar, I'll use my particular webmail client instead.

Hope this info helps,
Cheers,
Derek