feat(server): add fallback smtp config (#13377)

fix AF-2749

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Added support for configuring a fallback SMTP server for outgoing
emails.
* Introduced the ability to specify email domains that will always use
the fallback SMTP server.
* Enhanced email sending to automatically route messages to the
appropriate SMTP server based on recipient domain.

* **Documentation**
* Updated configuration options and descriptions in the admin interface
to reflect new fallback SMTP settings.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/core/mail/config.ts

@@ -1,3 +1,5 @@
+import z from 'zod';
+
 import { defineModuleConfig } from '../../base';
 
 declare global {
@@ -11,6 +13,16 @@ declare global {
         ignoreTLS: boolean;
         sender: string;
       };
+
+      fallbackDomains: ConfigItem<string[]>;
+      fallbackSMTP: {
+        host: string;
+        port: number;
+        username: string;
+        password: string;
+        ignoreTLS: boolean;
+        sender: string;
+      };
     };
   }
 }
@@ -46,4 +58,34 @@ defineModuleConfig('mailer', {
     default: false,
     env: ['MAILER_IGNORE_TLS', 'boolean'],
   },
+
+  fallbackDomains: {
+    desc: 'The emails from these domains are always sent using the fallback SMTP server.',
+    default: [],
+    shape: z.array(z.string()),
+  },
+  'fallbackSMTP.host': {
+    desc: 'Host of the email server (e.g. smtp.gmail.com)',
+    default: '',
+  },
+  'fallbackSMTP.port': {
+    desc: 'Port of the email server (they commonly are 25, 465 or 587)',
+    default: 465,
+  },
+  'fallbackSMTP.username': {
+    desc: 'Username used to authenticate the email server',
+    default: '',
+  },
+  'fallbackSMTP.password': {
+    desc: 'Password used to authenticate the email server',
+    default: '',
+  },
+  'fallbackSMTP.sender': {
+    desc: 'Sender of all the emails (e.g. "AFFiNE Team <noreply@affine.pro>")',
+    default: '',
+  },
+  'fallbackSMTP.ignoreTLS': {
+    desc: "Whether ignore email server's TSL certification verification. Enable it for self-signed certificates.",
+    default: false,
+  },
 });

packages/backend/server/src/core/mail/sender.ts

@@ -36,6 +36,8 @@ function configToSMTPOptions(
 export class MailSender {
   private readonly logger = new Logger(MailSender.name);
   private smtp: Transporter<SMTPTransport.SentMessageInfo> | null = null;
+  private fallbackSMTP: Transporter<SMTPTransport.SentMessageInfo> | null =
+    null;
   private usingTestAccount = false;
   constructor(private readonly config: Config) {}
 
@@ -61,11 +63,17 @@ export class MailSender {
   }
 
   private setup() {
-    const { SMTP } = this.config.mailer;
+    const { SMTP, fallbackDomains, fallbackSMTP } = this.config.mailer;
     const opts = configToSMTPOptions(SMTP);
 
     if (SMTP.host) {
       this.smtp = createTransport(opts);
+      if (fallbackDomains.length > 0 && fallbackSMTP?.host) {
+        this.logger.warn(
+          `Fallback SMTP is configured for domains: ${fallbackDomains.join(', ')}`
+        );
+        this.fallbackSMTP = createTransport(configToSMTPOptions(fallbackSMTP));
+      }
     } else if (env.dev) {
       createTestAccount((err, account) => {
         if (!err) {
@@ -83,21 +91,34 @@ export class MailSender {
     } else {
       this.logger.warn('Mailer SMTP transport is not configured.');
       this.smtp = null;
+      this.fallbackSMTP = null;
     }
   }
 
+  private getSender(domain: string) {
+    const { SMTP, fallbackSMTP, fallbackDomains } = this.config.mailer;
+    if (this.fallbackSMTP && fallbackDomains.includes(domain)) {
+      return [this.fallbackSMTP, fallbackSMTP.sender] as const;
+    }
+    return [this.smtp, SMTP.sender] as const;
+  }
+
   async send(name: string, options: SendOptions) {
-    if (!this.smtp) {
+    const [, domain, ...rest] = options.to.split('@');
+    if (rest.length || !domain) {
+      this.logger.error(`Invalid email address: ${options.to}`);
+      return null;
+    }
+
+    const [smtpClient, from] = this.getSender(domain);
+    if (!smtpClient) {
       this.logger.warn(`Mailer SMTP transport is not configured to send mail.`);
       return null;
     }
 
     metrics.mail.counter('send_total').add(1, { name });
     try {
-      const result = await this.smtp.sendMail({
-        from: this.config.mailer.SMTP.sender,
-        ...options,
-      });
+      const result = await smtpClient.sendMail({ from, ...options });
 
       if (result.rejected.length > 0) {
         metrics.mail.counter('rejected_total').add(1, { name });