What it means is, a mailserver should resolve local part case-insensitively, so that Bob@example.com and bob@example.com end up in the same mailbox.
But, to be spec-compliant, a mailserver MUST send on an email addressed to AlIcE@example.net to AlIcE@example.net, exactly, without downcasing it to alice@example.net.
I'd imagine this is often honored in the breach, but there you have it.
What it means is, a mailserver should resolve local part case-insensitively, so that Bob@example.com and bob@example.com end up in the same mailbox.
But, to be spec-compliant, a mailserver MUST send on an email addressed to AlIcE@example.net to AlIcE@example.net, exactly, without downcasing it to alice@example.net.
I'd imagine this is often honored in the breach, but there you have it.