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content/Docker/Common things with docker-mailserver.md

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+## 1. docker-mailserver SSL alert number 42
+
+ I was getting following error after setting up TLS certificates in docker-mailserver after setting it up with `letsencrypt`:
+
+```
+TLS handshaking: SSL_accept() failed: error:14094412:SSL routines:ssl3_read_bytes:sslv3 alert bad certificate: SSL alert number 42
+```
+
+The problem was caused by two reasons
+### 1.1. Letsencrypt and wildcard domains
+
+My `MX` domain was `example.com`, bu `letsencrypt` have been set up with wildcard certificate on `*.example.com`.
+
+So, specifying `SSL_MODE=letsencrypt` made it searching for `/etc/letsencrypt/mail.example.com`, when it was just `/etc/letsencrypt/example.com`
+
+**SOLUTION**: 
+
+Change .env file to:
+
+- `SSL_MODE=manual`
+- `SSL_KEY_PATH=/etc/letsencrypt/example.com/fullchain.pem`
+- `SSL_KEY_PATH=/etc/letsencrypt/example.com/privkey.pem`
+
+Don't forget to modify `docker-compose` volumes and pass certs from host filesystem. Note: they're not `mail.example.com`, they're just `example.com`.
+
+This [ticket](https://github.com/docker-mailserver/docker-mailserver/issues/1607) describes it well.
+
+### 1.2. Docker was using internal hostname of docker-mailserver
+
+Internal hostname for `docker-mailserver` was just `mail`, and other container tried to send emails to `mail:25`.
+
+**SOLUTION:** change internal hostname in your app settings to actual, specified in your certificates and MX record.
+
+### 2. docker-mailserver not listening on 25 port (SMTP)
+
+Solution was to add quotes in your docker compose, just like that:
+
+```yaml
+  ports:
+    - "25:25"
+```
+
+This [ticket](https://github.com/docker-mailserver/docker-mailserver/issues/684#issuecomment-322029794) solved it
+
+### 3. Connect to docker-mailserver from other docker-compose
+
+I've had `mailserver/compose.yaml` and `application/compose.yaml` and needed to connect `application` to `mailserver`.
+
+```yaml
+// application/compose.yaml
+app:
+  networks:
+    - shared
+networks:
+  shared:
+	driver: bridge
+```
+
+```yaml
+/// mailserver/compose.yaml
+mail:
+  networks:
+     - "application_shared"
+networks:
+  application_shared:
+    external: true
+```
+
+### 4. How to convert DKIM mail.txt to DNS record
+
+My `mail.txt` was like:
+
+```
+mail._domainkey IN      TXT     ( "v=DKIM1; h=sha256; k=rsa; "
+          "p=sOmEJuNkLiKeRaNdOmOrLikeThat"
+          "eVeNmOrERANdOmStuFf" )  ; ----- DKIM key mail for example.com
+```
+
+**SOLUTION**: concatenate all strings inside round brackets:
+
+```
+v=DKIM1; h=sha256; k=rsa; p=sOmEJuNkLiKeRaNdOmOrLikeThateVeNmOrERANdOmStuFf
+```
+
+And add them as a `TXT` record on my DNS provider control panel under `mail._domainkey` record.
+
+Solution was right in [official documentation](https://docker-mailserver.github.io/docker-mailserver/edge/config/best-practices/dkim_dmarc_spf/#dkim-dns), just hidden a little bit.