[samatman]

These are of course not in conflict.

What it means is, a mailserver should resolve local part case-insensitively, so that [e-mail] and [e-mail] end up in the same mailbox.

But, to be spec-compliant, a mailserver MUST send on an email addressed to [e-mail] to [e-mail], exactly, without downcasing it to [e-mail].

I'd imagine this is often honored in the breach, but there you have it.

[gowld]

"mail server" is ambiguous. SMTP Simple Mail Transfer Protocol is about transferring mail between domains, which must preserve the local part.

An MDA Mail Delivery Agent or LDA Local Delivery Agent inside a domain can choose whether to be case sensitive, but is discouraged from doing so.

The same issue exists for dots '.' in local part.