If someone does that with one of my (custom domain) addresses, it won’t work, here’s what I implemented: https://zackorndorff.com/2015/03/10/disposable-email-address...

(To save you a click, they look like aa_COMPANY+SHORTHASH@mydomain.com, with shorthash being based on COMPANY and a secret)

Downside is the address ends up absurdly long, and I’ve had to manually create some aliases for companies that won’t accept the plus.

I don’t recommend this setup, it’s kind of a pain to maintain, but I wish one of the mainstream providers would implement something similar.