Hey, this has turned out to be a very interesting thread. I'm learning a lot from this exchange.
Re identities:
It's been a long time since I examined how FM handles them. Spurred on by this thread, today I had a woman's look at the list of options on the Settings page in my FM account. As we all know, you always find something in the last place you look, and there it was, 'Sending Identities', at the bottom of the list of options in the right margin.
Yes, I have various sending identities created there automatically by my FM account. I've long since noticed they are there in the drop down menu on the Compose screen, but admit to using them infrequently. I think they'll get more use in future, especially with the strategic use of aliases.
I haven't yet had time to check the links on identities you provided, but a quick online search - fastmail identities - pulled up an FM documentation page with that very name as the first hit.
I hadn't before considered using catchall syntax when creating an identity. I confess that I have seen it mentioned in these forums on a few occasions over the years, but I never comprehended that I could use it in my account. I thought it had something to do with people adjusting incoming mail settings for their own email systems or domains. Thank you for demystifying that for me.
Considering the use of a catchall though, it will catch everything. That might create issues with spam management. Unless a rule you've created is applied before the catchall accepts delivery.
Anyhow, should there be a need in future for a special reply address that can be used in the From field of outgoing messages that I compose, I'll know how to create it. The interface for making it looks easy to use, obviously detailed understanding of its options will require a bit of learning investment.
Re making rules:
The rules screen of one's FM account in years gone by used to be more complicated than it is today. I think it is still fairly complex - which is good because that gives more power and versatility - but it used to be harder to use than now. Previously, coded syntax had to be used in the creation of rules, and I learnt what I needed to know by reading posts to these forums. In the current interface, rules are created by selecting from various options that are described in more or less plain English. I think the way it works is these plain language options apply the appropriate syntax to the FM back-end and effect the result you (hopefully) intended.
Another issue with the Rules screen of olden days was the way rules were saved after being created. Having just checked my stock of rules, it contains a number of duplicates and repeats. That's because I would make a rule, follow the syntax I had copied from examples found on these forums, and then the spam would keep on arriving. So I'd make another one, and still no result. It turned out there was a two stage save process, and I had overlooked the second stage. That issue was solved for me by fellow contributors to these forums. Thankfully, that part of the FM user interface experience has long been fixed.
I gave an example in the form of:
ivm@mrdavid.fmdomain.tld
I'm not sure which rules I made successfully blocked it, but these are the ones I have in my list:
1) "any header" "matches glob pattern" [ *ivm* ]
2) "a header called" [ To ] "contains" [ ivm ]
"a header called" [ Cc ] "contains" [ ivm ]
"a header called" [ X-Delivered-To ] "contains" [ ivm ]
The second rule has a number of conditions, probably because when the first rule was created I was not aware additional conditions could be made.