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Updated A mail server's 101 (markdown)

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Jean-Denis Vauguet 2020-06-10 00:23:41 +02:00
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@ -58,6 +58,20 @@ Similarly to IMAP, POP3 may be secured with either: _Implicit_ (enforced) TLS (a
**The best practice as of 2020 would be [POP3S](https://en.wikipedia.org/wiki/POP3S) over port 995**, rather than [POP3](https://en.wikipedia.org/wiki/POP3)+STARTTLS over port 110 (see [RFC 8314](https://tools.ietf.org/html/rfc8314)).
### Summary of ports/security setups
| Protocol | Opt-in Encryption<sup>1</sup> | Enforced Encryption | Purpose |
|----------|---------------------------------|---------------------|----------------------|
| SMTP | 25 | N/A | Transfer<sup>2</sup> |
| ESMTP | 587 _(deprecated<sup>4</sup>)_ | 465<sup>3</sup> | Submission |
| POP3 | 110 _(deprecated<sup>4</sup>)_ | 995 | Retrieval |
| IMAP4 | 143 _(deprecated<sup>4</sup>)_ | 993 | Retrieval |
1. A connection *may* be secured over TLS when _both_ ends support `STARTTLS`. On ports 110, 143 and 587, `docker-mailserver` will reject a connection that cannot be secured _(preventing [MITM attacks](https://stackoverflow.com/questions/15796530/what-is-the-difference-between-ports-465-and-587/32460763#32460763) trough a downgrading)_. Port 25 is [required](https://serverfault.com/questions/623692/is-it-still-wrong-to-require-starttls-on-incoming-smtp-messages) to support insecure connections.
2. Port 25 receives email and filters for spam and viruses. For submitting outgoing mail you should prefer the submission ports (465, 587), which require authentication in docker-mailserver. Unless a relay host is configured, outgoing email will _leave_ the server via port 25 (thus outbound traffic must not be blocked by your provider or firewall).
3. Port 465 is a submission port since 2018, see [RFC 8314](https://tools.ietf.org/html/rfc8314). Originally a secure variant of port 25, it is now dedicated to SMTPS.
4. [RFC 8314](https://tools.ietf.org/html/rfc8314) is recommending that clear text exchanges to be abandoned and that all three common IETF mail protocols to be used only in implicit mode (no STARTTLS).
## How does docker-mailserver help with setting everything up?
As a _batteries included_ Docker image, docker-mailserver provides you with all the required components and a default configuration to run a mail server. On top of that, the [env-mailserver](https://github.com/tomav/docker-mailserver/blob/master/env-mailserver.dist) configuration file (and some other optional, advanced files!) allow you to tweak your setup extensively. You may even derive your own image from docker-mailserver for a complete control.