Shopify also flags orders (even if they are fully paid) when the customer's email contains the shop's name. I'm sure their logic is that it helps cut down on fraud.
I prefer the system of using a basic abbreviation in the email address to avoid these types of filters but still make the email easily traceable later. Say your name is joe smith and you're buying from Sports Online. Something like Joe_spo_smith@yourdomain.com works well for later confirming whom you gave the address to.
I like this approach because one doesn't have to track all of the catch-all emails made on the fly, since finding out who you originally gave that email to is just a matter of searching your past email and noticing that "spo" looks more like those old Sports Online emails and not the new spam from discounted wholesale fancy rugs.
Apart from seeing who a company has on-sold your information to (or more rarely had a staff member steal their database), it's also an easy way to see who has been hacked.
One other reason not to use something like SportsOnline@yourdomain.com is that some websites exist merely as credential honeypots and those types are usually aware of this approach and will then typically exploit the catch-all for spamming. The shady All on MP3 service was known for doing this. (I'm pretty sure that site existed solely to exploit the fact that most people used the same password for everthing.)