sender check: From: read with OpenDKIM's own parser; L8 closed

Audit 2026-10-12, stage 2c, the owner's decision of 2026-10-14: the check
decides by the address OpenDKIM signs for, whatever a display name or a
comment holds.

internal/milter/mailparse.go ports libopendkim's dkim_mail_parse() with
dkim_mail_first_special(), dkim_mail_token() and
dkim_mail_matching_paren() from Debian's opendkim 2.11.0~beta2-8+deb12u1,
line for line: the '<' form, group and list punctuation, an '@' before the
first ':', ';' or ',' deciding where the address starts, source routes
dropped, quoted strings and domain literals kept with their delimiters,
comments skipped, whitespace between atoms one space, no encoded word
decoded; and the C's own quirks ("@d" gives d as the local part too).
testdata/mailparse_harness.c compiles the C from the image's source; the
test feeds it and the port 200 019 lines (the hand-written cases and
200 000 random ones over the specials) - 93 443 parse and agree byte for
byte, 106 576 fail and agree on the error (on an error the C leaves its
in-place result unterminated, and OpenDKIM uses neither part then).

judge(): one From: field and one address stay required (moreAfter: only
list or group punctuation may follow); what OpenDKIM cannot read is
refused (mlfi_eoh() would not sign it); the address is lower-cased in
ASCII only, as dkimf_lowercase() and Postfix's tables do. Found reading
dkimf_apply_signtable(): OpenDKIM matches its refile SigningTable against
"user@domain" printed into MAXADDRESS + 1 = 257 bytes, so a longer address
is matched cut short - a quoted local part padded to end there in
"@another.tenant" would be signed with that tenant's key while its real
domain is the application's. Such an address (over 256 bytes) is refused;
one that fits is matched whole. The display-name and comment scan, the
"one @ outside quotes" rule and the SigningTable reader (Signers) go; the
check asks for header values with their leading space as OpenDKIM does
(OptHeaderLeadingSpace; Postfix keeps a v2 milter's event mask, milter8.c
milter8_negotiate()).

L8 closed: refused or passed no longer depends on other domains of the
server; security.md's accepted risk is struck. The shared other-sender
lines are now held to "never signed for the other sender": unit test (a
passed line is read at the application's domain) and e2e (a passed line
arrives signed for the application's domain, none for the other).
Documents: architecture "How the header is read", security's sender
check paragraph, the guide; the help topic per 2cf118c.

Stand (2026-10-15, make e2e, exit 0): sender_check_holds_the_from_line -
another sender's address in the display name now passes; a From: whose
quoted local part is padded so that, cut at 256 bytes, it ends in
"@other.e2e.test" is refused 550 5.7.1 "the address is longer than
OpenDKIM reads whole"; lines_naming_another_sender_never_signed_for_it
and passed_from_lines_signed_for_their_domain_only pass. go vet and go
test ./... pass on the server (24 packages); the differential test ran
there against the compiled C.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/view/help.go

@@ -169,7 +169,7 @@ var outboundTopics = []HelpTopic{
 		Rich("An application sends only as itself, and that holds for both senders a message has. The envelope sender is held against the login by Postfix. The ",
 			Code("From:"), " line, the one a mail program shows and the one the DKIM signature is chosen by, is held against it by the panel, under the same rule: an application set to any address of its domain may write any address of that domain, and one with a list of addresses only those. A message with any other ",
 			Code("From:"), " - another of your domains, the server's own name, an address somewhere else, or none at all - is refused for good with ",
-			Code("550 5.7.1"), ". It is a rejected row of the Outbound log, and its page says which address was refused. This is what keeps one application from getting mail signed with another domain's key.",
+			Code("550 5.7.1"), ". The line is read the way the signer, OpenDKIM, reads it, so only its address counts: what a display name or a comment around it says is not judged, and a line holding more than one address is refused. It is a rejected row of the Outbound log, and its page says which address was refused. This is what keeps one application from getting mail signed with another domain's key.",
 			Br(), Br(),
 			"The check is strict. While the panel does not answer, Postfix does not accept mail unchecked: it replies with a temporary error and the application sends again later. Server › Health counts what that costs: for how long the panel did not answer while Postfix itself was running; how many connections Postfix turned away because of it - it refuses from the connection on, before a message is offered, so a client that tries three times is three and a monitor that only connects is one; and how many messages were refused with a temporary error in the middle, when the panel stopped answering after a message had begun. A time marked ≈ includes a start at which supervisord could not say since when Postfix was running: the time since the panel's last sign of life is counted, which may be more than Postfix was up. The card warns when a connection was turned away or a message refused in the last 24 hours. If the numbers stay at nothing, the check is where it should be; if they do not, it belongs in a process of its own."),
 	}},