added dovecot smtp proxy

.gitignore

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+.env
+data
+docker-compose.override.yml

README.md

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-# smtp-proxy
+# Dovecot SMTP Submission Proxy
+
+Dovecot deployment acting as a pure SMTP submission proxy, companion to [imap-proxy](../imap-proxy): it listens on your hostname (e.g. `smtp.example.com`) with its own SSL certificate, accepts and terminates the client connections (implicit TLS on 465, STARTTLS on 587), then connects to the destination server (e.g. `smtp.example.net`) as if it were the client, over the destination's own SSL, and forwards the traffic bit-by-bit in both directions.
+
+Your mail clients (Thunderbird, Apple Mail, the Gmail app, ...) are configured once against your hostname and certificate; which provider actually serves the mail is decided in one config line and can be changed at will — no need to buy the provider a certificate for your domain or pay for custom-hostname setups.
+
+## How it works
+Dovecot's login proxy is used with a `static` passdb: any username is accepted, no password is verified locally (`nopassword`), and the login is forwarded (credentials included) to the destination submission server, which performs the real authentication. Once the remote login succeeds, dovecot becomes a dumb pipe between client and destination. No mail is ever stored or queued locally: if the destination rejects a message, the client sees the rejection directly.
+
+Because the password must be forwarded, clients have to use cleartext mechanisms (`PLAIN` / `LOGIN`); dovecot only allows them after TLS is negotiated, so nothing travels unencrypted.
+
+Port 25 is intentionally not handled: it is server-to-server traffic driven by your MX records, which can point straight at the provider — no certificate for your client-facing hostname is involved there.
+
+## Configuration
+Copy `example.env` to `.env` and adjust:
+- `SSL_CERT` / `SSL_KEY`: certificate and key for the hostname the proxy serves (e.g. Let's Encrypt live paths).
+- `SUBMISSION_EXPOSE` / `SMTPS_EXPOSE`: host bindings for ports 587/465.
+
+Copy `example.dovecot.conf` to `${DOVECOT_CONF}` (default `./data/dovecot.conf`) and set `host` in the `passdb static` block to your destination submission server (port 465 with `ssl = yes`, or port 587 with `starttls = yes`). The destination certificate is verified against the system CA bundle (`ssl_client_ca_file`); use `ssl = any-cert` instead of `ssl = yes` if the destination has a self-signed certificate.
+
+## Usage
+```sh
+cp example.env .env
+mkdir -p data && cp example.dovecot.conf data/dovecot.conf
+$EDITOR .env data/dovecot.conf
+docker compose up -d
+```
+
+## Testing
+```sh
+openssl s_client -connect smtp.example.com:465 -quiet
+# then authenticate as an existing user on the destination server:
+EHLO test
+AUTH PLAIN <base64 of "\0user@example.com\0password">
+```
+(generate the AUTH string with `printf '\0user@example.com\0password' | base64`)
+
+## Notes
+The `-root` image flavor is used so dovecot can read certificates that are only readable by root (as the Let's Encrypt live folder is); dovecot itself still drops privileges per-service. If your certificates are world-readable you can drop the `-root` suffix from `DOVECOT_VERSION`.

docker-compose.yml

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+---
+services:
+  dovecot:
+    image: dovecot/dovecot:${DOVECOT_VERSION}
+    restart: unless-stopped
+    ports:
+      - ${SUBMISSION_EXPOSE}:587
+      - ${SMTPS_EXPOSE}:465
+    volumes:
+      - ${DOVECOT_CONF}:/etc/dovecot/dovecot.conf:ro
+      - ${SSL_CERT}:/etc/dovecot/ssl/cert.pem:ro
+      - ${SSL_KEY}:/etc/dovecot/ssl/key.pem:ro

example.dovecot.conf

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+# Dovecot SMTP submission proxy configuration (dovecot 2.4 syntax)
+#
+# Terminates SSL/TLS for the local hostname (e.g. smtp.example.com) and
+# proxies every login, credentials included, to the destination server
+# (e.g. smtp.example.net) over its own SSL/TLS. After the login succeeds,
+# dovecot forwards the connection bit-by-bit in both directions.
+
+dovecot_config_version = 2.4.0
+dovecot_storage_version = 2.4.0
+
+protocols = submission
+log_path = /dev/stdout
+
+ssl = yes
+ssl_server_cert_file = /etc/dovecot/ssl/cert.pem
+ssl_server_key_file = /etc/dovecot/ssl/key.pem
+ssl_min_protocol = TLSv1.2
+ssl_client_ca_file = /etc/ssl/certs/ca-certificates.crt
+
+auth_mechanisms = plain login
+
+service submission-login {
+  inet_listener submission {
+    port = 587
+  }
+  inet_listener submissions {
+    port = 465
+    ssl = yes
+  }
+}
+
+# Destination submission server. "ssl = yes" + port 465 = implicit TLS;
+# for a STARTTLS destination use port 587 with "starttls = yes" instead
+# of the ssl field.
+passdb static {
+  fields {
+    nopassword = yes
+    proxy = yes
+    host = smtp.example.net
+    port = 465
+    ssl = yes
+  }
+}

example.env

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+# Dovecot
+DOVECOT_VERSION=2.4.4-root
+DOVECOT_CONF=./data/dovecot.conf
+SUBMISSION_EXPOSE=587
+SMTPS_EXPOSE=465
+
+# SSL certificates
+SSL_CERT=/etc/letsencrypt/live/smtp.example.com/fullchain.pem
+SSL_KEY=/etc/letsencrypt/live/smtp.example.com/privkey.pem