[xyzzy]

Can someone explain how to create and use wildcard aliases or verify what I describe here? I did some experimenting and I think either I'm doing it wrong, missing something in the setup, or thinking there's more "power" in these than I am giving them credit for.

Here's how I created a wildcard alias in the settings (actual names and domains changed for the example here):

1. Created an alias in Aliases. Called it myAlias@123mail.org, delivered to my actual FastMail email address. This creates an identity in Identities & Fetch. Note, I originally thought you create the wildcard alias in Aliases but the settings don't allow that. They want a real name, not a '*'.
2. In Identities & Fetch I add a new identity and name it *@123mail.org with a reply-to set to myAlias@123mail.org.

At this point I can compose an email, select *@123mail.org in the compose account choices which allows me to create any name I want in front of @123mail.org, for example example@123mail.org. I can send it and receive it showing that From is example@123mail.org. If I reply to it it goes to the name example@123mail.org but actually to myAlias@123mail.org.

Now here is where my confusion starts. Assuming I set up the wildcard alias correctly (did I?) what if someone knows my email address by the fake address example@123mail.org and they try to send to it? This is not a reply just some place using that fake email address. That's where I'm confused because obviously that isn't going to work.

A mail sent by me using example@123mail.org has the true reply-to specified from the identity, i.e., the original myAlias@123mail.org alias and identity. So a reply has a valid place to be sent. But just the sender using the example@123mail.org as recipient address that isn't a reply has no valid place to go and will bounce.

That's the reason for this post. Am I giving too much credit to the power of wildcard aliases or did I make a wrong turn in setting these things up in the first place? The intent was to supply different @123mail.org addresses to different places I need to register or specify an email address so I could tell when those places distributed it and who distributed it when they attempt to send me email using the fake address. That's what I thought was implied in the FastMail Catchall/Wildcard Alias article (more about that article later).

If I am not doing anything wrong then I see no advantage to wildcard aliases and might as well just use a single distinct alias/identities and filter on those email addresses. I was hoping the wildcard alias would mean I don't have to do that.

So, bottom line, I hope I'm doing something wrong creating the wildcard alias. If so, what is it?

Back to that FastMail Catchall/Wildcard Alias article. It's instructions for creating an wildcard alias are either out of date or wrong. They say,

1. Go to Settings → Aliases
2. Create a New Alias
3. Set the email address to be *@<domain.tld>
4. Optional: update any other options as required. See the aliases page for more information.
5. Save

And it's step 3 that appears wrong and why I did what I did to create the wildcard alias. I cannot create an alias as described in step 2. That's how I ended up having to create a wildcard address in Identites&Fetch.