Those who use iCloud mail, how is spam filtering?
It seems to work okay, but it's generally pretty opaque. You sort of have to trust that it's working. I guess that's no different than Gmail, but based on some early experiences I never had much confidence in it. It may have improved since then in quality, but it's still a black box in terms of how it works.
Then again, I prefer services like Fastmail where I can at least see things like spam scores and adjust thresholds to fine tune the system. iCloud naturally offers none of that.
Does it support rules for processing incoming mail and automatically tagging/marking read/etc?
Sort of. You can mark read, file into folders, move to trash, and forward emails. You can't set flags on incoming messages, nor can you create specific auto-reply rules — there's only a single generic "Vacation Rule" that auto-replies to everybody within a certain timeframe. Rules can also be based on from/to/cc/subject and list-id, but they can't search the body of an incoming email, nor act on things like attachments or whether the sender is in your address book.
Also, the rules have to be configured directly in the iCloud web interface — they don't sync to Apple Mail on the Mac. It has its own rules, which are much more sophisticated, but are only processed locally on the Mac. This isn't too bad if you have a Mac running all the time with the Mail app open, since it can run those rules locally, which will sync to your other devices, but it's still not as great as server-side rules.
Any idea whether the iCloud mail custom domain support will allow multiple custom domains or just the one? Catch-all addresses?
To be clear, all the hype and discussion about custom domain support has been based on this one paragraph found on
Apple's iOS 15 Preview Page:
Custom email domain
Personalize your iCloud Mail address with a custom domain name, and invite family members to use the same domain with their iCloud Mail accounts.
That is the entirety of what we know about this upcoming feature right now. However, if I had to guess, I'd say that knowing Apple, it's probably going to be limited to a single custom domain, which is hinted at by the use of the singular in that paragraph ("a custom domain name") and the fact that it's clearly geared toward creating a single domain that you can invite your other family members to. I strongly suspect this will also be tied to Family Sharing, since Apple has to provide a way for you to control who else can use addresses at your domain, and there's no point in creating a whole new interface when the current Family Sharing arrangement will do the job.
I also wouldn't hold my breath on support for things like catch-all addresses. After all, we only need to look at how relatively basic iCloud Mail is to know that it's never been about power user features.
Of course, I'm also hopeful that maybe Apple will use this opportunity to beef up iCloud+ Mail, and if they're taking that approach, then anything's possible.