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Update documentation on port 25 blocking

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Expand the `README.md` a bit to better explain what's needed to get the server up and running.

It seems that a lot of users are trying to use this image and running into issues. This should help out a bit to get them started.
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Boky 2020-03-16 08:34:21 +01:00 committed by GitHub
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README.md
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@ -25,8 +25,16 @@ You can now send emails by using `localhost:1587` as your SMTP server address. O
you haven't configured your `example.com` domain to allow sending from this IP (see
[openspf](http://www.openspf.org/)), your emails will most likely be regarded as spam.
All standard caveats of configuring the SMTP server apply -- e.g. you'll need to make sure your DNS
entries are updated properly if you don't want your emails marked as spam.
All standard caveats of configuring the SMTP server apply:
- **MAKE SURE YOUR OUTGOING PORT 25 IS NOT BLOCKED.**
- Most ISPs block outgoing connections to port 25 and several companies (e.g. [NoIP](https://www.noip.com/blog/2013/03/26/my-isp-blocks-smtp-port-25-can-i-still-host-a-mail-server/), [Dynu](https://www.dynu.com/en-US/Blog/Article?Article=How-to-host-email-server-if-ISP-blocks-port-25) offer workarounds).
- Hosting centers also tend to block port 25, which can be unblocked per requirst (e.g. for AWS either [fill out a form](https://aws.amazon.com/premiumsupport/knowledge-center/ec2-port-25-throttle/) or forward mail to their [SES](https://aws.amazon.com/ses/) service, which is free for low volumes)
- You'll most likely need to at least [set up SPF records](https://en.wikipedia.org/wiki/Sender_Policy_Framework) or [DKIM](https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail)
- If using DKIM (below), make sure to add DKIM keys to your domain's DNS entries
- You'll most likely need to set up (PTR)[https://en.wikipedia.org/wiki/Reverse_DNS_lookup] records to prevent your mails going to spam
If you don't know what any of the above means, get some help. Google is your friend. It's also worth noting that as a consequence it's pretty difficult to host a SMTP server on a dynamic IP address.
**Please note that the image uses the submission (587) port by default**. Port 25 is not
exposed on purpose, as it's regularly blocked by ISP or already occupied by other services.