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See also Zimbra Self-Signed SSL Certs
Howto: https://wiki.zimbra.com/wiki/Installing_a_LetsEncrypt_SSL_Certificate
If you have trouble reissuing a new cert, or if Zimbra won't start, recreate and deploy a new self-signed cert to get Zimbra 'working' again. Then re-implement a LetsEncrypt cert.
If a cert is expired, you must reissue a new cert.
If a certificate renewal fails, try reissuing a new cert instead.