diff --git a/src/lib-storage/index/index-mail.c b/src/lib-storage/index/index-mail.c
index 05be89c1a6..5f4f8a43e9 100644
--- a/src/lib-storage/index/index-mail.c
+++ b/src/lib-storage/index/index-mail.c
@@ -456,7 +456,7 @@ bool index_mail_get_cached_virtual_size(struct index_mail *mail, uoff_t *size_r)
 
 	/* see if we can get it from index */
 	mail_index_lookup_ext(_mail->transaction->view, _mail->seq,
-			      mail->vsize_ext_id, &idata, &expunged);
+			      _mail->box->mail_vsize_ext_id, &idata, &expunged);
 	const uint32_t *vsize = idata;
 
 	if (vsize != NULL && *vsize > 0) {
@@ -489,7 +489,7 @@ bool index_mail_get_cached_virtual_size(struct index_mail *mail, uoff_t *size_r)
 	    data->virtual_size < (uint32_t)-1) {
 		uint32_t vsize = data->virtual_size+1;
 		mail_index_update_ext(_mail->transaction->itrans, _mail->seq,
-				      mail->vsize_ext_id, &vsize, NULL);
+				      _mail->box->mail_vsize_ext_id, &vsize, NULL);
 	}
 
 	return TRUE;
@@ -876,7 +876,7 @@ static void index_mail_cache_sizes(struct index_mail *mail)
 		extension for box virtual size exists
 	*/
 
-	if ((mail_index_map_get_ext_idx(view->map, mail->vsize_ext_id, &idx) ||
+	if ((mail_index_map_get_ext_idx(view->map, _mail->box->mail_vsize_ext_id, &idx) ||
 	     mail_index_map_get_ext_idx(view->map, _mail->box->vsize_hdr_ext_id, &idx)) &&
 	    (sizes[0] != (uoff_t)-1 &&
 	     sizes[0] < (uint32_t)-1)) {
@@ -887,7 +887,7 @@ static void index_mail_cache_sizes(struct index_mail *mail)
 		vsize = sizes[0] + 1;
 		sizes[0] = (uoff_t)-1;
 		mail_index_update_ext(_mail->transaction->itrans, _mail->seq,
-				      mail->vsize_ext_id, &vsize, NULL);
+				      _mail->box->mail_vsize_ext_id, &vsize, NULL);
 	}
 
 	for (i = 0; i < N_ELEMENTS(size_fields); i++) {
@@ -1540,9 +1540,6 @@ void index_mail_init(struct index_mail *mail,
 	mail->mail.v = *t->box->mail_vfuncs;
 	mail->mail.mail.box = t->box;
 	mail->mail.mail.transaction = t;
-	mail->vsize_ext_id = mail_index_ext_register(t->box->index, "vsize", 0,
-						     sizeof(uint32_t),
-						     sizeof(uint32_t));
 	t->mail_ref_count++;
 	mail->mail.data_pool = pool_alloconly_create("index_mail", 16384);
 	mail->ibox = INDEX_STORAGE_CONTEXT(t->box);
@@ -2208,15 +2205,15 @@ void index_mail_precache(struct mail *mail)
 }
 
 static void
-index_mail_reset_vsize_ext(struct mail *mail, struct index_mail *imail)
+index_mail_reset_vsize_ext(struct mail *mail)
 {
 	unsigned int idx;
 	uint32_t vsize = 0;
 	struct mail_index_view *view = mail->transaction->view;
-	if (mail_index_map_get_ext_idx(view->map, imail->vsize_ext_id,
+	if (mail_index_map_get_ext_idx(view->map, mail->box->mail_vsize_ext_id,
 				       &idx)) {
 		mail_index_update_ext(mail->transaction->itrans, mail->seq,
-				      imail->vsize_ext_id, &vsize, NULL);
+				      mail->box->mail_vsize_ext_id, &vsize, NULL);
 	}
 }
 
@@ -2236,14 +2233,14 @@ void index_mail_set_cache_corrupted(struct mail *mail,
 		imail->data.physical_size = (uoff_t)-1;
 		imail->data.virtual_size = (uoff_t)-1;
 		imail->data.parts = NULL;
-		index_mail_reset_vsize_ext(mail, imail);
+		index_mail_reset_vsize_ext(mail);
 		break;
 	case MAIL_FETCH_VIRTUAL_SIZE:
 		field_name = "virtual size";
 		imail->data.physical_size = (uoff_t)-1;
 		imail->data.virtual_size = (uoff_t)-1;
 		imail->data.parts = NULL;
-		index_mail_reset_vsize_ext(mail, imail);
+		index_mail_reset_vsize_ext(mail);
 		break;
 	case MAIL_FETCH_MESSAGE_PARTS:
 		field_name = "MIME parts";
diff --git a/src/lib-storage/index/index-mail.h b/src/lib-storage/index/index-mail.h
index 99b65dc587..7daf60837d 100644
--- a/src/lib-storage/index/index-mail.h
+++ b/src/lib-storage/index/index-mail.h
@@ -137,7 +137,6 @@ struct index_mail {
 	struct index_mailbox_context *ibox;
 
 	int pop3_state;
-	uint32_t vsize_ext_id;
 
 	/* per-mail variables, here for performance reasons: */
 	uint32_t header_seq;
diff --git a/src/lib-storage/index/index-storage.c b/src/lib-storage/index/index-storage.c
index d55d7501f7..c3b91ddbf7 100644
--- a/src/lib-storage/index/index-storage.c
+++ b/src/lib-storage/index/index-storage.c
@@ -303,6 +303,9 @@ int index_storage_mailbox_open(struct mailbox *box, bool move_to_memory)
 	box->box_last_rename_stamp_ext_id =
 		mail_index_ext_register(box->index, "last-rename-stamp",
 					sizeof(uint32_t), 0, sizeof(uint32_t));
+	box->mail_vsize_ext_id = mail_index_ext_register(box->index, "vsize", 0,
+							 sizeof(uint32_t),
+							 sizeof(uint32_t));
 
 	box->opened = TRUE;
 
diff --git a/src/lib-storage/mail-storage-private.h b/src/lib-storage/mail-storage-private.h
index 69bbf18da1..614663d0c3 100644
--- a/src/lib-storage/mail-storage-private.h
+++ b/src/lib-storage/mail-storage-private.h
@@ -363,6 +363,7 @@ struct mailbox {
 	uint32_t pop3_uidl_hdr_ext_id;
 	uint32_t box_name_hdr_ext_id;
 	uint32_t box_last_rename_stamp_ext_id;
+	uint32_t mail_vsize_ext_id;
 
 	/* MAIL_RECENT flags handling */
 	ARRAY_TYPE(seq_range) recent_flags;