diff --git a/application/backend/src/controllers/MailerController.test.ts b/application/backend/src/controllers/MailerController.test.ts
index 86d41ac7..57f1e4f0 100644
--- a/application/backend/src/controllers/MailerController.test.ts
+++ b/application/backend/src/controllers/MailerController.test.ts
@@ -92,3 +92,33 @@ describe('MailerController', () => {
     })
   })
 })
+
+// Test due to nodemailer moving to more strict sender address parsing
+describe('Mailer Configuration', () => {
+  const originalHostname = process.env.HOSTNAME
+
+  beforeEach(() => {
+    jest.resetModules()
+  })
+
+  afterEach(() => {
+    process.env.HOSTNAME = originalHostname
+  })
+
+  it('should strip http:// and ports from HOSTNAME in fromAddress', async () => {
+    // non RFC-compliant environment variable
+    process.env.HOSTNAME = 'http://localhost:5173'
+
+    const mailerModule = await import('../utils/mailer')
+
+    expect(mailerModule!.fromAddress).toBe('CTRL <noreply@localhost>')
+  })
+
+  it('should handle plain domains without protocols', async () => {
+    process.env.HOSTNAME = 'production-domain.com'
+
+    const mailerModule = await import('../utils/mailer')
+
+    expect(mailerModule!.fromAddress).toBe('CTRL <noreply@production-domain.com>')
+  })
+})
diff --git a/application/backend/src/utils/mailer.ts b/application/backend/src/utils/mailer.ts
index 23534c1d..0b1e0e0e 100644
--- a/application/backend/src/utils/mailer.ts
+++ b/application/backend/src/utils/mailer.ts
@@ -1,7 +1,24 @@
 import nodemailer from 'nodemailer'
 import config from '../config'
 
-export const fromAddress = `CTRL <noreply@${process.env.HOSTNAME}>`
+if (!process.env.HOSTNAME) {
+  throw new Error('process.env.HOSTNAME is required but was not provided.')
+}
+
+const hostnameEnvVar = process.env.HOSTNAME
+let mailDomain: string
+
+if (/^https?:\/\//i.test(hostnameEnvVar)) {
+  try {
+    mailDomain = new URL(hostnameEnvVar).hostname
+  } catch (err: any) {
+    throw new Error(`HOSTNAME deployment variable not configured. ${err}`)
+  }
+} else {
+  mailDomain = hostnameEnvVar
+}
+
+export const fromAddress = `CTRL <noreply@${mailDomain}>`
 
 export async function createMailerTransporter() {
   if (process.env.STUB_MAILER == 'true') {