I'm still doing some binary-style testing of the various notification / forwarding options in respect of Calendar, and I'll report back when I'm clear as to what works and what doesn't.....
On Keef's point above, that approach will work, but it will only work if you want ALL the mail form the Calendar-associated GMail address to be diverted / picked up elsewhere. For assorted reasons, that won't work for me - I just want to control the destination of Calendar items, especially the Daily Agenda.
Another global solution that I guess would work - prompted by Rainboe's 4 Calendars, above.... - is to create a new GMail address that is used solely for Calendar business, and then to forward all mail from that address to the external address......
BTW, the GMail Spam filter is indeed very effective - but it does conceal any non-spam that's wrongly marked as spam pretty well....... There's also an issue with GMail and its dislike for what it reads as duplicate messages. For instance, if I set American Airlines to update me on gates and times for a flight, the mail that AA sends out has a) identical subject info, and b) the varied contents as an attachment. I've found that GMail will acept the very first notification, but then filters out all subsequent updates as being "identical repeats", unless I've deleted the original notification.... Kind of defeats the purpose of wanting serial updates.....
AGB