Mimeparser uses unprotected Date header

[link2xt]

We already protect the Date header against modification here:

core/src/mimefactory.rs

Lines 914 to 936 in e5a3eae

	} else if is_encrypted {
	protected_headers.push(header.clone());
	match header_name.as_str() {
	"subject" => {
	unprotected_headers.push((
	"Subject",
	mail_builder::headers::raw::Raw::new("[...]").into(),
	));
	}
	"date"
	| "in-reply-to"
	| "references"
	| "auto-submitted"
	| "chat-version"
	| "autocrypt-setup-message" => {
	unprotected_headers.push(header.clone());
	}
	_ => {
	// Other headers are removed from unprotected part.
	}
	}
	} else {

There is an open PR #6640 to protect the Date by replacing it with unix epoch in the outer header.

However, it cannot be merged because mimeparser uses unprotected timestamp before trying to decrypt the message:

core/src/mimeparser.rs

Lines 234 to 235 in e5a3eae

	let mut timestamp_sent =
	Self::get_timestamp_sent(&mail.headers, timestamp_rcvd, timestamp_rcvd);