The SMTP connection between n8n and IONOS is not working

Questions
1

Problem: Unable to connect IONOS SMTP with n8n Cloud - getting “535 Authentication credentials invalid” error, despite credentials working perfectly in Thunderbird.

Environment:

  • n8n Cloud (not self-hosted) - Running version [email protected]

  • IONOS email account with domain

  • Email Send (SMTP) node

What I’ve already tried:

  1. Port 587 with STARTTLS:

    • Host: smtp.ionos.com

    • Port: 587

    • User: (full email address)

    • Password: the webmail password (confirmed correct)

    • SSL/TLS: OFF

    • Disable STARTTLS: OFF

  2. Port 465 with SSL/TLS:

    • Same credentials as above

    • Port: 465

    • SSL/TLS: ON

  3. Various authentication formats:

    • Full email address as username ✓

    • Verified password multiple times ✓

    • Checked for app-specific passwords (IONOS doesn’t use them for regular email)

  4. Verified IONOS requirements:

    • Domain matching (From Email = authentication email)

Important note: The exact same SMTP settings work flawlessly in Thunderbird and other email clients, confirming the credentials are correct. This appears to be an n8n Cloud-specific compatibility issue with IONOS SMTP servers.

And IONOS does not require creating a separate app password.

Question: Is there a known compatibility issue between n8n Cloud and IONOS SMTP? Are there any workarounds besides using third-party SMTP relay services?

2

Hello DavidP, thank you for the question. I’ll try to help you here yaa.

This looks less like bad credentials and more like how IONOS enforces SMTP auth with cloud providers.

Maybe there are several suggestions that you can try:

  1. Use port 587 + STARTTLS (IONOS’s documented preferred). Set the configuration like this:

In n8n Cloud, make sure “From Email” matches the authenticated email exactly (IONOS rejects if From didnt same with login)

  1. Forces auth method (node in n8n doent expose it directly). A workaround is to use the SMTP credentials in a Send Email node via smtp-transport or wrapp them in a Postmark/SendGrid relay if step 1 keeps failing.
  2. You can try to check SPF/DMARC on your domain, even through auth should pass, IONOS sometimes silently fails is SPF doesnt align when the sender is a cloud service. Just make sure your DNS TXT record includes include:_spf.perfora.net include:_spf.kundenserver.de
  3. The last maybe you can try to do test outside n8n Cloud. Spin up the same node locally (npx n8n) and point it at IONOS. If its works locally but fails on Cloud, its specific to Cloud infra, likely blocked by IONOS.

Maybe that is suggest you can try, and i hope that will be the solution for you. :blush::raising_hands: