This is true… but RFC 5321 says the local part is case SENSITIVE, and further, that no assumptions should be made about the installation and configuration options set on the mailhandling machine.
From the RFC:
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.
The fact that it is handled as though case insensitive in many configurations and set ups doesn’t change the fact that applications accessing and using the server are explicitly told that they CANNOT ASSUME THAT. And given that discourse does use those email addresses to mail, they have no choice but to assume case sensitivity if they are using email address as log on.
