in the docs
https://docker-mailserver.github.io/docker-mailserver/edge/config/security/ssl/#caddy
under 
Caddy v2 JSON example snippet
this part is wrong:

"tls": {
      "automation": {
        "policies": [
          {
            "subjects": [
              "mail.example.com",
            ],
            "key_type": "rsa2048",
            "issuer": {
              "email": "[email protected]",
              "module": "acme"
            }
          },
          {
            "issuer": {
              "email": "[email protected]",
              "module": "acme"
            }
          }
        ]
      }

https://caddyserver.com/docs/json/apps/tls/
says no issuer field
there is only a issuers field under policies, so it shoud be

,
    "tls": {
      "automation": {
        "policies": [
          {
            "subjects": [
              "mail.example.com"
            ],
            "key_type": "rsa2048"
          }
        ]
      }
    }