Providers
How well an address checks out depends mostly on who runs the mailbox, not on who does the checking.
Yahoo and AOL say yes to every address on purpose. It stops people harvesting their user list. Gmail and Microsoft 365 block checks from cloud servers, and slow down the rest.
So these pages say which providers we read well, and which ones nobody can read.
Run a sample through it
100 free checks a month, no card, and the unknowns come back labeled.