static int check_mailbox(struct Mailbox *m)
{
  if (!m)
    return -1;

  struct NntpMboxData *mdata = m->mdata;
  struct NntpAccountData *adata = mdata->adata;
  time_t now = mutt_date_epoch();
  int rc = 0;
  void *hc = NULL;

  if (adata->check_time + C_NntpPoll > now)
    return 0;

  mutt_message(_("Checking for new messages..."));
  if (nntp_newsrc_parse(adata) < 0)
    return -1;

  adata->check_time = now;
  int rc2 = nntp_group_poll(mdata, false);
  if (rc2 < 0)
  {
    nntp_newsrc_close(adata);
    return -1;
  }
  if (rc2 != 0)
    nntp_active_save_cache(adata);

  /* articles have been renumbered, remove all headers */
  if (mdata->last_message < mdata->last_loaded)
  {
    for (int i = 0; i < m->msg_count; i++)
      email_free(&m->emails[i]);
    m->msg_count = 0;
    m->msg_tagged = 0;

    if (mdata->last_message < mdata->last_loaded)
    {
      mdata->last_loaded = mdata->first_message - 1;
      if (C_NntpContext && (mdata->last_message - mdata->last_loaded > C_NntpContext))
        mdata->last_loaded = mdata->last_message - C_NntpContext;
    }
    rc = MUTT_REOPENED;
  }

  /* .newsrc has been externally modified */
  if (adata->newsrc_modified)
  {
#ifdef USE_HCACHE
    unsigned char *messages = NULL;
    char buf[16];
    struct Email *e = NULL;
    anum_t first = mdata->first_message;

    if (C_NntpContext && (mdata->last_message - first + 1 > C_NntpContext))
      first = mdata->last_message - C_NntpContext + 1;
    messages = mutt_mem_calloc(mdata->last_loaded - first + 1, sizeof(unsigned char));
    hc = nntp_hcache_open(mdata);
    nntp_hcache_update(mdata, hc);
#endif

    /* update flags according to .newsrc */
    int j = 0;
    for (int i = 0; i < m->msg_count; i++)
    {
      if (!m->emails[i])
        continue;
      bool flagged = false;
      anum_t anum = nntp_edata_get(m->emails[i])->article_num;

#ifdef USE_HCACHE
      /* check hcache for flagged and deleted flags */
      if (hc)
      {
        if ((anum >= first) && (anum <= mdata->last_loaded))
          messages[anum - first] = 1;

        snprintf(buf, sizeof(buf), "%u", anum);
        struct HCacheEntry hce = mutt_hcache_fetch(hc, buf, strlen(buf), 0);
        if (hce.email)
        {
          bool deleted;

          mutt_debug(LL_DEBUG2, "#1 mutt_hcache_fetch %s\n", buf);
          e = hce.email;
          e->edata = NULL;
          deleted = e->deleted;
          flagged = e->flagged;
          email_free(&e);

          /* header marked as deleted, removing from context */
          if (deleted)
          {
            mutt_set_flag(m, m->emails[i], MUTT_TAG, false);
            email_free(&m->emails[i]);
            continue;
          }
        }
      }
#endif

      if (!m->emails[i]->changed)
      {
        m->emails[i]->flagged = flagged;
        m->emails[i]->read = false;
        m->emails[i]->old = false;
        nntp_article_status(m, m->emails[i], NULL, anum);
        if (!m->emails[i]->read)
          nntp_parse_xref(m, m->emails[i]);
      }
      m->emails[j++] = m->emails[i];
    }

#ifdef USE_HCACHE
    m->msg_count = j;

    /* restore headers without "deleted" flag */
    for (anum_t anum = first; anum <= mdata->last_loaded; anum++)
    {
      if (messages[anum - first])
        continue;

      snprintf(buf, sizeof(buf), "%u", anum);
      struct HCacheEntry hce = mutt_hcache_fetch(hc, buf, strlen(buf), 0);
      if (hce.email)
      {
        mutt_debug(LL_DEBUG2, "#2 mutt_hcache_fetch %s\n", buf);
        if (m->msg_count >= m->email_max)
          mx_alloc_memory(m);

        e = hce.email;
        m->emails[m->msg_count] = e;
        e->edata = NULL;
        if (e->deleted)
        {
          email_free(&e);
          if (mdata->bcache)
          {
            mutt_debug(LL_DEBUG2, "mutt_bcache_del %s\n", buf);
            mutt_bcache_del(mdata->bcache, buf);
          }
          continue;
        }

        m->msg_count++;
        e->read = false;
        e->old = false;
        e->edata = nntp_edata_new();
        e->edata_free = nntp_edata_free;
        nntp_edata_get(e)->article_num = anum;
        nntp_article_status(m, e, NULL, anum);
        if (!e->read)
          nntp_parse_xref(m, e);
      }
    }
    FREE(&messages);
#endif

    adata->newsrc_modified = false;
    rc = MUTT_REOPENED;
  }

  /* some headers were removed, context must be updated */
  if (rc == MUTT_REOPENED)
    mailbox_changed(m, NT_MAILBOX_INVALID);

  /* fetch headers of new articles */
  if (mdata->last_message > mdata->last_loaded)
  {
    int oldmsgcount = m->msg_count;
    bool verbose = m->verbose;
    m->verbose = false;
#ifdef USE_HCACHE
    if (!hc)
    {
      hc = nntp_hcache_open(mdata);
      nntp_hcache_update(mdata, hc);
    }
#endif
    int old_msg_count = m->msg_count;
    rc2 = nntp_fetch_headers(m, hc, mdata->last_loaded + 1, mdata->last_message, false);
    m->verbose = verbose;
    if (rc2 == 0)
    {
      if (m->msg_count > old_msg_count)
        mailbox_changed(m, NT_MAILBOX_INVALID);
      mdata->last_loaded = mdata->last_message;
    }
    if ((rc == 0) && (m->msg_count > oldmsgcount))
      rc = MUTT_NEW_MAIL;
  }

#ifdef USE_HCACHE
  mutt_hcache_close(hc);
#endif
  if (rc)
    nntp_newsrc_close(adata);
  mutt_clear_error();
  return rc;
}