Spam via xxx@keybase.io email addresses

Well that sucks! I just got two spams via my keybase.io email forwarding account. Was wondering how long that would take.

I rely 1% on a private blacklist and 99% on DNSBL lists and occasional use of spamcop.net (which has it's own DNSBL) to filter spam and it works almost flawlessly. Unfortunately, keybase.io would not want to get listed in either.

That said, maybe it could _use_ them to get started and see if the problem could be nipped in the bud. Here's relevant postfix config from my install:

```
smtpd_recipient_restrictions =
permit_mynetworks,
permit_sasl_authenticated,
check_client_access cidr:/etc/postfix/rbl_override # for persistent assholes
check_sender_access hash:/etc/postfix/sender_access,
reject_non_fqdn_hostname,
reject_non_fqdn_sender,
reject_non_fqdn_recipient,
reject_unauth_pipelining,
reject_invalid_hostname,
reject_rbl_client b.barracudacentral.org,
reject_rbl_client bl.spamcop.net,
reject_rbl_client dnsbl.sorbs.net,
reject_rbl_client cbl.abuseat.org,
reject_rbl_client zen.spamhaus.org
```

Maybe this kind of thing should be set up on keybase.io?

No repository file, test, or entry point is identified; begin by confirming whether keybase.io email forwarding is handled in this repository and reviewing the proposed Postfix DNSBL configuration. Done would require an agreed spam-filtering approach and evidence that it prevents abusive forwarded mail without blocking legitimate messages.

Written by the indexing model from the issue text.