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.)

> I.e. foo.tld will refuse sending email to mailbox+foo@mydomain.tld.

While I never had that issue, I had Foo’s legal service contact me about using their trademarked name in my email (a short explanation cleared it up)

Some mail providers support receiving mail on arbitrary hostnames, so you can set up a wildcard MX record and then use mailbox@foo.example.com instead. This avoids email validation issues with plus addresses, spammers don’t try removing any parts of the hostname, and I think in the many years I have been using it I’ve only run into a problem with including the service name once or possibly twice.

Some MTAs (Postfix at least) allows other characters. I use an underscore, which seems universally accepted. For the second problem, there's usually some alternative mangling that would still be unique-enough, or just flee because they seem too aggressive on the data harvesting.

On Fastmail, at least, this isn't a big issue. For a catchall account, if I reply to an email[0] it automatically addresses the reply as from the alias. It is editable in place, too, in case I want to give it some other name.

[0] In their webmail client, of course

But why would you not be able to reply with the aliased email? I do this regularly. Of course your mail client needs to support this, but using Mutt this is absolutely no problem (just change the FROM header) and I have heard from other users who use Thunderbird that they can also create new identities (just not as easy on the fly). In fact I wrote a small script for Mutt that would automatically set the correct sender if I reply to an email that contained a wildcard. Works pretty well.

Can you not reply from a user+foo@example.com alias? I use the catchall approach (so just foo@example.com when signing up for foo), but if I need to email customer support I'll just send the email from foo@example.com. I've never tried that with a + in the account though to see if my client supports it.

Dots are useful to create new accounts on the same site, but you can’t use it to just tell websites apart (because there’s no label)