Could this be an extension of that ? Looks like.
SMTP RFC 2821 says that the local part of the email address is case sensitive. Some ignore this and consider upper and lower case the same.
Section 2.4:
> The local-part of a mailbox MUST BE treated as case sensitive. Therefore, SMTP implementations MUST take care to preserve the case of mailbox local-parts. Mailbox domains are not case sensitive.
> However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged.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.
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.
I've certainly strtolower'ed some email inputs...