diff --git a/src/lib-imap/imap-bodystructure.c b/src/lib-imap/imap-bodystructure.c
index 88f865d8ef..0b5b3e8606 100644
--- a/src/lib-imap/imap-bodystructure.c
+++ b/src/lib-imap/imap-bodystructure.c
@@ -473,7 +473,7 @@ static void part_write_body(const struct message_part *part,
 	part_write_bodystructure_common(data, str);
 }
 
-bool imap_bodystructure_is_plain_7bit(const struct message_part *part)
+bool message_part_data_is_plain_7bit(const struct message_part *part)
 {
 	const struct message_part_data *data = part->data;
 
diff --git a/src/lib-imap/imap-bodystructure.h b/src/lib-imap/imap-bodystructure.h
index 7a24156109..b3d54594ab 100644
--- a/src/lib-imap/imap-bodystructure.h
+++ b/src/lib-imap/imap-bodystructure.h
@@ -32,9 +32,9 @@ void message_part_data_parse_from_header(pool_t pool,
 	struct message_part *part,
 	struct message_header_line *hdr);
 
-/* Returns TRUE if BODYSTRUCTURE is
-   ("text" "plain" ("charset" "us-ascii") NIL NIL "7bit" n n NIL NIL NIL) */
-bool imap_bodystructure_is_plain_7bit(const struct message_part *part)
+/* Returns TRUE if this message part has content-type "text/plain",
+   chaset "us-ascii" and content-tranfer-encoding "7bit" */
+bool message_part_data_is_plain_7bit(const struct message_part *part)
 	ATTR_PURE;
 
 /* Write a BODY/BODYSTRUCTURE from given message_part. The message_part->data
diff --git a/src/lib-storage/index/index-mail.c b/src/lib-storage/index/index-mail.c
index 8a2e1b2791..616c687df4 100644
--- a/src/lib-storage/index/index-mail.c
+++ b/src/lib-storage/index/index-mail.c
@@ -644,7 +644,7 @@ static void index_mail_body_parsed_cache_flags(struct index_mail *mail)
 						_mail->seq, cache_flags_idx);
 
 	if (data->parsed_bodystructure &&
-	    imap_bodystructure_is_plain_7bit(data->parts) &&
+	    message_part_data_is_plain_7bit(data->parts) &&
 	    (want_cached || want_plain_bodystructure_cached(mail))) {
 		cache_flags |= MAIL_CACHE_FLAG_TEXT_PLAIN_7BIT_ASCII;
 		/* we need message_parts cached to be able to