I am running 10.6.8 and I don't have an iCloud icon in the System Preferences, so how am I suppose to upgrade my iCloud storage?

I completely agree. All of our important business communications are sent and received with MacMail, although the other addresses and their repective and respectable storage have proved to be angst free. Also important is that several "free" mail domains are supported with advertising and charges for additional storage.

I meant that I use a professional mail system, one with retention and tracking facilities (so that if an e-mail fails to arrive I can find out where it is and why it was not delivered, then have it delivered.) These facilities (especially retention) do not exist on consumer mail systems at all (as far as I have searched)

I use Microsoft Exchange, $15 per month and I have never lost an e-mail, outgoing or incoming. I also have 10 years of retention, so I can always access old emails when required (like when the IRS want to see some from 2008) I have found that "I can't find it" is not good enough for the authorities.

The mail client in use (Mail, Thunderbird, Outlook etc) is not relevant.