Add example configuration

.env.example

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+# Mailroom example configuration
+
+# ----------------------------
+# Outbox (Submission) Server Configuration
+# ----------------------------
+OUTBOX_PORT=465
+OUTBOX_HOST=smtp.example.com
+
+# ----------------------------
+# Inbox (MX) Server Configuration
+# ----------------------------
+INBOX_PORT=25
+INBOX_HOST=mail.example.com
+
+# Required authentication for incoming mail.
+# Allowed values: spf dkim arc dmarc
+# If left blank, authentication is optional.
+INBOX_AUTH="spf dkim dmarc"
+
+# ----------------------------
+# Global TLS Configuration
+# ----------------------------
+TLS_CIPHERS=TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256
+TLS_MIN_VERSION=TLSv1.3
+TLS_MAX_VERSION=TLSv1.3
+
+# ----------------------------
+# Inbox TLS Configuration
+# ----------------------------
+INBOX_TLS_KEY_PATH=/certs/inbox/privkey.pem
+INBOX_TLS_CERT_PATH=/certs/inbox/cert.pem
+
+# ----------------------------
+# Outbox TLS Configuration
+# ----------------------------
+OUTBOX_TLS_KEY_PATH=/certs/outbox/privkey.pem
+OUTBOX_TLS_CERT_PATH=/certs/outbox/cert.pem
+
+# ----------------------------
+# Client Certificate Configuration
+# ----------------------------
+# If requiring client certificates for IMAP/Outbox,
+# Specify the Certificate Authority and path for user certificates
+CLIENT_CERT_CA_PATH=/certs/clients/ca-cert.pem
+CLIENT_CERT_PATH=/certs/clients/users/
+
+# ----------------------------
+# Mailbox Configuration
+# ----------------------------
+MAILBOX_PATH=/var/mail/vhosts/
+
+# ----------------------------
+# Rspamd Configuration
+# ----------------------------
+RSPAMD_PASSWORD=""