diff --git a/src/lib-mail/rfc2231-parser.c b/src/lib-mail/rfc2231-parser.c
index e940b9492f..9b6cd4d360 100644
--- a/src/lib-mail/rfc2231-parser.c
+++ b/src/lib-mail/rfc2231-parser.c
@@ -47,8 +47,14 @@ int rfc2231_parse(struct rfc822_parser_context *ctx,
 	string_t *str;
 	unsigned int i, j, count, next, next_idx;
 	bool ok, have_extended, broken = FALSE;
+	char prev_replacement_char;
 	int ret;
 
+	/* Temporarily replace the nul_replacement_char while we're parsing
+	   the content-params. It'll be restored before we return. */
+	prev_replacement_char = ctx->nul_replacement_char;
+	ctx->nul_replacement_char = RFC822_NUL_REPLACEMENT_CHAR;
+
 	/* Get a list of all parameters. RFC 2231 uses key*<n>[*]=value pairs,
 	   which we want to merge to a key[*]=value pair. Save them to a
 	   separate array. */
@@ -97,6 +103,7 @@ int rfc2231_parse(struct rfc822_parser_context *ctx,
 			array_append(&result, &value, 1);
 		}
 	}
+	ctx->nul_replacement_char = prev_replacement_char;
 
 	if (array_count(&rfc2231_params_arr) == 0) {
 		/* No RFC 2231 parameters */
diff --git a/src/lib-mail/rfc2231-parser.h b/src/lib-mail/rfc2231-parser.h
index 1c44349880..0db536fd4e 100644
--- a/src/lib-mail/rfc2231-parser.h
+++ b/src/lib-mail/rfc2231-parser.h
@@ -3,8 +3,9 @@
 
 /* Parse all content parameters using rfc822_parse_content_param() and return
    them as a NULL-terminated [key, value] array. RFC 2231-style continuations
-   are merged to a single key. Returns -1 if some of the input was invalid
-   (but valid key/value pairs are still returned), 0 if everything looked ok. */
+   are merged to a single key. NULs are converted into 0x80. Returns -1 if some
+   of the input was invalid (but valid key/value pairs are still returned), 0
+   if everything looked ok. */
 int ATTR_NOWARN_UNUSED_RESULT
 rfc2231_parse(struct rfc822_parser_context *ctx,
 	      const char *const **result_r);
diff --git a/src/lib-mail/test-rfc2231-parser.c b/src/lib-mail/test-rfc2231-parser.c
index 59204f4ee5..3c7e1b61f7 100644
--- a/src/lib-mail/test-rfc2231-parser.c
+++ b/src/lib-mail/test-rfc2231-parser.c
@@ -7,18 +7,18 @@
 
 static void test_rfc2231_parser(void)
 {
-	const char *input =
+	const unsigned char input[] =
 		"; key4*=us-ascii''foo"
 		"; key*2=ba%"
 		"; key2*0=a"
 		"; key3*0*=us-ascii'en'xyz"
-		"; key*0=\"foo\""
+		"; key*0=\"f\0oo\""
 		"; key2*1*=b%25"
 		"; key3*1=plop%"
 		"; key*1=baz";
 	const char *output[] = {
 		"key",
-		"foobazba%",
+		"f\x80oobazba%",
 		"key2*",
 		"''ab%25",
 		"key3*",
@@ -32,10 +32,10 @@ static void test_rfc2231_parser(void)
 	unsigned int i;
 
 	test_begin("rfc2231 parser");
-	rfc822_parser_init(&parser, (const void *)input, strlen(input), NULL);
+	rfc822_parser_init(&parser, input, sizeof(input)-1, NULL);
 	test_assert(rfc2231_parse(&parser, &result) == 0);
 	for (i = 0; output[i] != NULL && result[i] != NULL; i++)
-		test_assert(strcmp(output[i], result[i]) == 0);
+		test_assert_idx(strcmp(output[i], result[i]) == 0, i);
 	rfc822_parser_deinit(&parser);
 	test_assert(output[i] == NULL && result[i] == NULL);
 	test_end();