submission: Add automatic BCC support for relayed messages

Add per-user configurable BCC functionality to the submission service.
When submission_bcc is set to a valid email address, all messages
relayed through the submission service will have a copy sent to that
address.

New settings:
- submission_bcc: BCC recipient address (empty = disabled)
- submission_bcc_ignore_errors: If FALSE (default), the DATA command
  fails when the relay rejects the BCC recipient. If TRUE, delivery
  proceeds with a warning logged.

The BCC recipient is added before sending DATA. In strict mode
(submission_bcc_ignore_errors=no), the server waits for the BCC
RCPT response and fails the transaction if rejected.

Author: andrewachen

src/submission/submission-backend-relay.c

@@ -6,6 +6,7 @@
 #include "settings.h"
 #include "mail-user.h"
 #include "iostream-ssl.h"
+#include "smtp-address.h"
 #include "smtp-client.h"
 #include "smtp-client-connection.h"
 #include "smtp-client-transaction.h"
@@ -655,6 +656,55 @@ struct relay_cmd_data_context {
 	struct smtp_server_transaction *trans;
 };
 
+static void
+relay_cmd_data_callback(const struct smtp_reply *relay_reply,
+			struct relay_cmd_data_context *data_ctx);
+
+struct relay_cmd_bcc_context {
+	struct submission_backend_relay *backend;
+	struct smtp_server_cmd_ctx *cmd;
+	struct smtp_server_transaction *trans;
+	struct relay_cmd_data_context *data_ctx;
+	struct istream *data_input;
+	bool ignore_errors;
+};
+
+static void
+relay_cmd_bcc_rcpt_callback(const struct smtp_reply *relay_reply,
+			    struct relay_cmd_bcc_context *bcc_ctx)
+{
+	struct submission_backend_relay *rbackend = bcc_ctx->backend;
+	struct submission_backend *backend = &rbackend->backend;
+	struct smtp_reply reply;
+
+	if (!backend_relay_handle_relay_reply(rbackend, bcc_ctx->cmd,
+					      relay_reply, &reply))
+		return;
+
+	if (!smtp_reply_is_success(&reply)) {
+		if (bcc_ctx->ignore_errors) {
+			e_warning(backend->event,
+				  "BCC recipient rejected (ignored): %s",
+				  smtp_reply_log(&reply));
+			/* best-effort: proceed with DATA despite BCC failure */
+			smtp_client_transaction_send(
+				rbackend->trans, bcc_ctx->data_input,
+				relay_cmd_data_callback, bcc_ctx->data_ctx);
+		} else {
+			e_error(backend->event, "BCC recipient rejected: %s",
+				smtp_reply_log(&reply));
+			/* strict mode: fail the DATA command */
+			smtp_server_reply(bcc_ctx->cmd, 550, "5.5.0",
+				"BCC recipient rejected by relay");
+		}
+		return;
+	}
+
+	/* BCC accepted: proceed with DATA */
+	smtp_client_transaction_send(rbackend->trans, bcc_ctx->data_input,
+				     relay_cmd_data_callback, bcc_ctx->data_ctx);
+}
+
 static void
 relay_cmd_data_rcpt_callback(const struct smtp_reply *relay_reply,
 			     struct submission_recipient *srcpt)
@@ -775,6 +825,7 @@ backend_relay_cmd_data(struct submission_backend *backend,
 {
 	struct submission_backend_relay *rbackend =
 		container_of(backend, struct submission_backend_relay, backend);
+	struct client *client = backend->client;
 	struct relay_cmd_data_context *data_ctx;
 
 	/* start relaying to relay server */
@@ -788,6 +839,47 @@ backend_relay_cmd_data(struct submission_backend *backend,
 
 	backend_relay_cmd_data_init_callbacks(rbackend, trans);
 
+	/* Add BCC recipient if configured */
+	if (client->set->submission_bcc[0] != '\0') {
+		struct smtp_address *bcc_addr;
+		const char *error;
+
+		if (smtp_address_parse_path(
+			trans->pool, client->set->submission_bcc,
+			SMTP_ADDRESS_PARSE_FLAG_ALLOW_LOCALPART,
+			&bcc_addr, &error) < 0) {
+			/* This shouldn't happen if settings validation passed,
+			   but handle it gracefully by skipping BCC */
+			e_error(backend->event,
+				"Invalid BCC address '%s': %s",
+				client->set->submission_bcc, error);
+		} else {
+			struct relay_cmd_bcc_context *bcc_ctx;
+
+			bcc_ctx = p_new(trans->pool,
+					struct relay_cmd_bcc_context, 1);
+			bcc_ctx->backend = rbackend;
+			bcc_ctx->cmd = cmd;
+			bcc_ctx->trans = trans;
+			bcc_ctx->data_ctx = data_ctx;
+			bcc_ctx->data_input = data_input;
+			bcc_ctx->ignore_errors =
+				client->set->submission_bcc_ignore_errors;
+
+			smtp_client_transaction_add_pool_rcpt(
+				rbackend->trans, trans->pool, bcc_addr, NULL,
+				relay_cmd_bcc_rcpt_callback, bcc_ctx);
+
+			e_debug(backend->event, "Added BCC recipient: %s",
+				smtp_address_encode(bcc_addr));
+
+			/* Defer send() until BCC RCPT callback - it will
+			   either proceed with DATA or fail the command
+			   depending on BCC result and strict/best-effort mode */
+			return 0;
+		}
+	}
+
 	smtp_client_transaction_send(rbackend->trans, data_input,
 				     relay_cmd_data_callback, data_ctx);
 	return 0;

src/submission/submission-settings.c

@@ -7,6 +7,7 @@
 #include "master-service-settings.h"
 #include "service-settings.h"
 #include "mail-storage-settings.h"
+#include "smtp-address.h"
 #include "submission-settings.h"
 
 #include <unistd.h>
@@ -84,6 +85,9 @@ static const struct setting_define submission_setting_defines[] = {
 	DEF(IN_PORT, submission_relay_port),
 	DEF(BOOL, submission_relay_trusted),
 
+	DEF(STR, submission_bcc),
+	DEF(BOOL, submission_bcc_ignore_errors),
+
 	DEF(STR, submission_relay_user),
 	DEF(STR, submission_relay_master_user),
 	DEF(STR, submission_relay_password),
@@ -127,6 +131,9 @@ static const struct submission_settings submission_default_settings = {
 	.submission_relay_port = 25,
 	.submission_relay_trusted = FALSE,
 
+	.submission_bcc = "",
+	.submission_bcc_ignore_errors = FALSE,
+
 	.submission_relay_user = "",
 	.submission_relay_master_user = "",
 	.submission_relay_password = "",
@@ -234,6 +241,21 @@ submission_settings_verify(void *_set, pool_t pool ATTR_UNUSED, const char **err
 	}
 	if (*set->hostname == '\0')
 		set->hostname = p_strdup(pool, my_hostdomain());
+
+	if (set->submission_bcc[0] != '\0') {
+		struct smtp_address *bcc_addr;
+		const char *error;
+
+		if (smtp_address_parse_path(
+			pool, set->submission_bcc,
+			SMTP_ADDRESS_PARSE_FLAG_ALLOW_LOCALPART,
+			&bcc_addr, &error) < 0) {
+			*error_r = t_strdup_printf(
+				"Invalid submission_bcc address '%s': %s",
+				set->submission_bcc, error);
+			return FALSE;
+		}
+	}
 #endif
 	return TRUE;
 }

src/submission/submission-settings.h

@@ -38,6 +38,10 @@ struct submission_settings {
 	in_port_t submission_relay_port;
 	bool submission_relay_trusted;
 
+	/* submission BCC: */
+	const char *submission_bcc;
+	bool submission_bcc_ignore_errors;
+
 	const char *submission_relay_user;
 	const char *submission_relay_master_user;
 	const char *submission_relay_password;