また次のメールが届いた．

Let’s Encrypt certificate expiration notice for domain “itl.mydns.jp” (and 4 more)

Hello,
Your certificate (or certificates) for the names listed below will expire in 20 days (on 30 Aug 19 02:53 +0000). Please make sure to renew your certificate before then, or visitors to your website will encounter errors.
We recommend renewing certificates automatically when they have a third of their total lifetime left. For Let’s Encrypt’s current 90-day certificates, that means renewing 30 days before expiration. See https://letsencrypt.org/docs/integration-guide/ for details.
itl.mydns.jp
…
mish.myds.me
For any questions or support, please visit https://community.letsencrypt.org/. Unfortunately, we can’t provide support by email.
For details about when we send these emails, please visit https://letsencrypt.org/docs/expiration-emails/. In particular, note that this reminder email is still sent if you’ve obtained a slightly different certificate by adding or removing names. If you’ve replaced this certificate with a newer one that covers more or fewer names than the list above, you may be able to ignore this message.
If you are receiving this email in error, unsubscribe at http://mandrillapp.com/track/unsub.php?u=30850198&id=fab156addb7b4097869d6fbc1603e9d3.RsjpgIgXcXbPKK4%2Fg4vK9vppkUw%3D&r=https%3A%2F%2Fmandrillapp.com%2Funsub%3Fmd_email%3Diwao.n%2540ae.auone-net.jp
Regards,
The Let’s Encrypt Team

Synology NAS の DSM で更新してくれるはずなのにうまく動作しなかったのか？
DSM の「コントロールパネル」－「セキュリティ」－「証明書」から対象のものを選んで「右クリック」－「証明書を更新」．

2021/07 NAS の証明書の更新