wip(smtp): How to use it

src/.vuepress/config.js

@@ -165,6 +165,13 @@ module.exports = {
             'export-import/project-service-export-import'
           ]
         },
+        {
+          title: 'SMTP',
+          collapsable: false,
+          children: [
+            'smtp/emails'
+          ]
+        },
         {
           title: 'Logs',
           collapsable: false,
@@ -403,6 +410,13 @@ module.exports = {
             'export-import/project-service-export-import'
           ]
         },
+        {
+          title: 'SMTP',
+          collapsable: false,
+          children: [
+            'smtp/emails'
+          ]
+        },
         {
           title: 'Logs',
           collapsable: false,

src/documentation/build/build-config.md

@@ -62,7 +62,7 @@ List the Zerops technologies your build uses as the **Zerops build base image**
 
 #### Supported base images
 
-##### `nodejs@<14, 16>`
+##### `nodejs@<14, 16, 18>`
 
 Includes `npm`, `yarn`, `git`, `npx`.
 

src/documentation/smtp/emails.md

@@ -0,0 +1,30 @@
+# SMTP configuration and email sending
+
+The default configuration of any Zerops project [private network](/documentation/routing/routing-between-project-services.html) blocks the SMTP 25 port from sending emails. The reason is primarily security and preventive protection against potential misuse of the offered [$20 credit](/documentation/overview/pricing.html#credit-and-free-zerops-credit) for the initial trial of Zerops for free for anyone.
+
+In practice, all **outgoing TCP ports 1 - 1024 except 22, 80, and 443**, and also all **outgoing UDP ports 1 - 65635** are blocked by the project firewall. This setting, therefore, eliminates using of the SMTP port 25 for outgoing email messages.
+
+## Zerops project's FW setting
+
+```yaml
+tcp dport 22 accept
+tcp dport 80 accept
+tcp dport 443 accept
+
+tcp dport 1-1024 drop
+udp dport 1-65535 drop
+```
+
+You will be able to use the SMTP port 25 without restrictions when your Zerops account is paid, but not when using a free credit. Actually, it would be best if you asked for this unblocking the Zerops team through our [Discord forum](https://discord.com/invite/WDvCZ54) or by sending an email to [[email protected]](mailto:[email protected]).
+
+<!-- markdownlint-disable DOCSMD004 -->
+::: tip Using an IPv4 address
+You'll be able to send emails over an IPv4 address even if you don't have anyone assigned to your project. In such a case, our NAT server will be used.
+:::
+<!-- markdownlint-enable DOCSMD004 -->
+
+<!-- markdownlint-disable DOCSMD004 -->
+::: tip Using a shadow SMTP server
+To be able to simulate SMTP traffic inside your project's network when developing or testing your applications, it's possible to install a shadow SMTP server as a service directly in your Zerops project. There are several possibilities in the OSS world on how to do it, and one of them is the [mailhog](https://github.com/mailhog/MailHog) project. You can read more about it in the article [Build a Ghost site in Zerops in a matter of minutes](https://medium.com/@mlapis/build-a-ghost-site-in-zerops-in-a-matter-of-minutes-e387a6da4b62).
+:::
+<!-- markdownlint-enable DOCSMD004 -->