Quote

If you use:

FEATURE(`blacklist_recipients')

then you can add entries to the map for local users, hosts in your
domains, or addresses in your domain which should not receive mail:

To:badlocaluser@ ERROR:550 Mailbox disabled for badlocaluser
To:host.my.TLD ERROR:550 That host does not accept mail
To:user@other.my.TLD ERROR:550 Mailbox disabled for this recipient

This would prevent a recipient of badlocaluser in any of the local
domains (class {w}), any user at host.my.TLD, and the single address
user@other.my.TLD from receiving mail. Please note: a local username
must be now tagged with an @ (this is consistent with the check of
the sender address, and hence it is possible to distinguish between
hostnames and usernames). Enabling this feature will keep you from
sending mails to all addresses that have an error message or REJECT
as value part in the access map. Taking the example from above:

spammer@aol.com REJECT
cyberspammer.com REJECT

Mail can't be sent to spammer@aol.com or anyone at cyberspammer.com.
That's why tagged entries should be used.