Happy to. @meta is the signer key for meta's cold-storage identity, the identity itself is @npub1u280z... (longer story, for a NIP write-up see below). Regular Nostr clients only ever show the signer. To actually see the identity, its profile and posts etc., log into www.inkan.cc with your NIP-7 extension. The client should show you a welcome popup with a list of identities you can follow that includes @npub1u280z..., so you can follow it and then go to its profile page to see its events and interact with it.

So I think what meta wants is the mailstr address on the master identity, with everyday login staying on the signer. That should be doable, the delegation between the two keys is on-chain, you'd read it and attribute the session up. Emails sent by the identity will probably also want OTS timestamps so it's checkable the delegation was live at the time the email was sent. I haven't really dug into how mailstr, but making it delegation-aware would be a great thing.

Here are more details, and I'm always happy to explain further:

naddr1qvzq...