[massaman_yams]

This won't detect a catch-all address, but will almost always detect a catch-all domain: send email to [long random string]@domain. Does it bounce or is it accepted?

[conquistadog]

How does the existence of a catch-all, of any kind, make it not a "valid business domain?"

[massaman_yams]

I'm not commenting on whether it's a good heuristic or not. Just a possible approach.

[yjftsjthsd-h]

Why would you block by that? I've worked places that intentionally did use the catch-all, mostly against misspellings.

[massaman_yams]

I use catchalls for my business domains too.