Add email address handling strategy

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 <https://migadu.com/>
 
+## A strategy to handle email addresses
+
+Email addresses can be "vanity" or "non-vanity".
+Vanity email addresses are meant to be public and memorable, for example `firstname@lastname.tld`.
+
+Set up any vanity domains that you want, and a non-vanity domain.
+
+### Non-vanity email addresses
+
+In the non-vanity domain, you create the `{me}{code}@nonvanity.tld` mailbox.
+
+`{me}` identifies you, you can have multiple users with different `{me}` identifiers in a single non-vanity domain.
+
+This strategy uses `{code}` to obfuscate email addresses.
+When you use `{code}` in an email address, you should be able to identify if the `{code}` is valid or not.
+
+For example, you could use a four-digit `{code}` and store what code you have used for each address.
+If you use `x3452` and store this code, when you receive an email that does not match, such as `x3453`, you know the code is incorrect.
+
+Alternatively, you can use hashing so that you do not have to store all codes.
+
+No one except you should know about `{me}{code}@nonvanity.tld`.
+
+Then you create a pattern rewrite from `{me}.*@nonvanity.tld` to `{me}{code}@nonvanity.tld`.
+
+When you need a non-vanity email address, you create a new `{me}.{entity}{code}@nonvanity.tld`, where `{entity}` is the entity that communicates with this email address and `{code}` is a **new** code.
+
+Mails received at `{me}@nonvanity.tld` are incorrect.
+Mails received without the correct code are incorrect.
+
+### Vanity email addresses
+
+Create any needed `{id}@vanity.tld` addresses.
+
+Different from non-vanity email addresses, vanity email addresses can be guessed and you cannot identify invalid email.
+
+TODO: set forwarding from `{id}@vanity.tld` to `{me}.{vanityid}{code}@nonvanity.tld`.
+
+### TODO Filing
+
+Because each vanity email address and entity has a different email address, you can file emails automatically into folders if wanted.
+
 ## Notes
 
 * Aliases do *not* have plus addressing, use a "pattern rewrite" instead.