There is one surprisingly good feature in yahoo mail that I wish google would provide: the ability (requirement, actually) to create a custom prefix for disposable email addresses. Just pick something completely unrelated to your real username. You can only create it once and it is forever your prefix, so don't mess up this step! Also, unlike gmail, you have to register a suffix when you want a new address in the form of [email protected]. But this has the added benefit that you can delete any of the suffixes from the list, thereby stopping incoming messages from that address. You can have up to 500 active suffixes.

Finally, since the format uses a dash rather than +, I have never once had a disposable yahoo address rejected.

Not sure if that made any sense, but here's a link to the docs:

https://help.yahoo.com/kb/SLN28338.html

Why would they care enough to do so?

If I get a marked as spam notification, I'll strip anything after the '+' (and really any special characters that distract from the raw email address) to make sure I never send an email to the address again.

Yes, this is part of standard data cleaning practices. You always remove whatever custom formats a provider allows to get the raw minimum email address (this is only for export, you still send official communication with a user to their original specified address).

There's a simple solution for that: always include a label as part of your address, and treat any mail that doesn't include one as spam.

Not every service even allows you to have a label in your email address. So what then?

Get them to fix their broken systems? The concept of a "label" exists only in the user agent. To everyone else the '+' character is just part of the account identifier, and a perfectly valid element of any e-mail address. The entire portion before the '@' sign is potentially significant. If someone hands you an address [email protected], there is no guarantee whatsoever that mail sent to [email protected] will end up in the same individual's inbox—that could be a completely unrelated account.

On the e-mail provider's side, one way to work around such broken systems would be to let users generate random, opaque aliases which do not contain their primary e-mail username but route to the same account, with a predetermined label applied on receipt. These would look just like ordinary addresses and there would be no way for services to strip out the identifying elements.