fix(api): add custom headers to email fallback SMTP (#3507)

fluxer_api/src/api/Config.ts

@@ -307,7 +307,10 @@ export function buildAPIConfigFromMaster(master: MasterConfig): APIConfig {
 			fallback: master.integrations.email.fallback
 				? {
 						domains: master.integrations.email.fallback.domains,
-						smtp: toSmtpConfig(master.integrations.email.fallback.smtp),
+						smtp: {
+							...toSmtpConfig(master.integrations.email.fallback.smtp),
+							headers: master.integrations.email.fallback.smtp.headers,
+						},
 					}
 				: undefined,
 		},

fluxer_api/src/api/config/APIConfig.ts

@@ -173,7 +173,7 @@ export interface APIConfig {
 		smtp?: SmtpConfig;
 		fallback?: {
 			domains: Array<string>;
-			smtp: SmtpConfig;
+			smtp: SmtpConfig & {headers?: Record<string, string>};
 		};
 	};
 	blocklistFeeds: {

fluxer_api/src/api/email/EmailProviderFactory.test.ts

@@ -4,7 +4,8 @@ import type {APIConfig} from '@app/api/config/APIConfig';
 import {createEmailProvider} from '@app/api/email/EmailProviderFactory';
 import {DomainRoutedEmailProvider} from '@pkgs/email/src/DomainRoutedEmailProvider';
 import {SmtpEmailProvider} from '@pkgs/email/src/SmtpEmailProvider';
-import {describe, expect, it} from 'vitest';
+import nodemailer, {type Transporter} from 'nodemailer';
+import {describe, expect, it, vi} from 'vitest';
 
 const smtp = {host: 'smtp.example.com', port: 587, username: 'user', password: 'pass', tlsMode: null};
 
@@ -33,6 +34,18 @@ describe('createEmailProvider', () => {
 		expect(provider).toBeInstanceOf(DomainRoutedEmailProvider);
 	});
 
+	it('adds the fallback headers only to mail through the fallback', async () => {
+		const sendMail = vi.fn();
+		vi.spyOn(nodemailer, 'createTransport').mockReturnValue({sendMail} as unknown as Transporter);
+		const headers = {'X-Tracking': '{"enable":0}'};
+		const provider = createEmailProvider(emailConfig({fallback: {domains: ['gmail.com'], smtp: {...smtp, headers}}}));
+		const from = {email: '[email protected]', name: 'Example'};
+		await provider?.sendEmail({to: '[email protected]', from, subject: 'Hi', text: 'Body'});
+		await provider?.sendEmail({to: '[email protected]', from, subject: 'Hi', text: 'Body'});
+		expect(sendMail.mock.calls[0][0].headers).toEqual({'X-Tracking': {prepared: true, value: '{"enable":0}'}});
+		expect(sendMail.mock.calls[1][0]).not.toHaveProperty('headers');
+	});
+
 	it.each([{enabled: false}, {provider: 'none' as const}, {smtp: undefined}])(
 		'returns no provider for %j even with a fallback',
 		(overrides) => {