added dovecot imap proxy

.gitignore

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

README.md

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-# imap-proxy
+# Dovecot IMAP Proxy
+
+Dovecot deployment acting as a pure IMAP proxy: it listens on your hostname (e.g. `imap.example.com`) with its own SSL certificate, accepts and terminates the client connections (implicit TLS on 993, STARTTLS on 143), then connects to the destination server (e.g. `imap.example.net`) as if it were the client, over the destination's own SSL, and forwards the traffic bit-by-bit in both directions.
+
+## 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 host, which performs the real authentication. Once the remote login succeeds, dovecot becomes a dumb pipe between client and destination. No mail is ever stored locally.
+
+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.
+
+## 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).
+- `IMAP_EXPOSE` / `IMAPS_EXPOSE`: host bindings for ports 143/993.
+
+Copy `example.dovecot.conf` to `${DOVECOT_CONF}` (default `./data/dovecot.conf`) and set `host` in the `passdb static` block to your destination server. The destination certificate is verified against the system CA bundle (`ssl_client_ca_file`); use `ssl = any-cert` instead of `ssl = yes` if the destination has a self-signed certificate.
+
+## Usage
+```sh
+cp example.env .env
+mkdir -p data && cp example.dovecot.conf data/dovecot.conf
+$EDITOR .env data/dovecot.conf
+docker compose up -d
+```
+
+## Testing
+```sh
+openssl s_client -connect imap.example.com:993 -quiet
+# then login as an existing user on the destination server:
+a LOGIN user@example.com password
+b LIST "" "*"
+```
+
+## 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:
+      - ${IMAP_EXPOSE}:143
+      - ${IMAPS_EXPOSE}:993
+    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 IMAP proxy configuration (dovecot 2.4 syntax)
+#
+# Terminates SSL/TLS for the local hostname (e.g. imap.example.com) and
+# proxies every login, credentials included, to the destination server
+# (e.g. imap.example.net) over its own SSL/TLS connection. 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 = imap
+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 imap-login {
+  inet_listener imap {
+    port = 143
+  }
+  inet_listener imaps {
+    port = 993
+    ssl = yes
+  }
+}
+
+passdb static {
+  fields {
+    nopassword = yes
+    proxy = yes
+    host = imap.example.net
+    port = 993
+    ssl = yes
+  }
+}

example.env

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+# Dovecot
+DOVECOT_VERSION=2.4.4-root
+DOVECOT_CONF=./data/dovecot.conf
+IMAP_EXPOSE=143
+IMAPS_EXPOSE=993
+
+# SSL certificates
+SSL_CERT=/etc/letsencrypt/live/imap.example.com/fullchain.pem
+SSL_KEY=/etc/letsencrypt/live/imap.example.com/privkey.pem