SAP Knowledge Base Article - Public

3769507 - Outbound emails bounced with SMTP 5.7.26 (unauthorized sender)

Symptom

Outbound emails are bounced (SMTP status code 5.7.26) despite relaxed DMARC alignment and maintained DNS records.

Environment

SAP Service Cloud Version 2

Reproducing the Issue

No specific steps to reproduce were provided.

Cause

The receiving server rejected the message with SMTP 5.7.26 because the sender was unauthenticated.

Resolution

  1. Verify SPF: Confirm the SPF DNS record authorizes the outbound sending server/IP and that the envelope sender (Mail-From/Return-Path) passes SPF.
  2. Verify DKIM: Ensure outbound emails are DKIM-signed and the corresponding DKIM selector DNS record is correctly published and validates.
  3. Confirm DMARC: Ensure the DMARC record is correctly published. Even with relaxed alignment, DMARC requires SPF or DKIM to pass.
  4. Configure a custom Mail-From domain for the email channel to improve authentication handling as per product guidance.
  5. Check receiving domain restrictions: Ask the receiving domain administrator to review any blocklist/restriction rules that may classify the sender domain or the service as unauthorized.
  6. Retest outbound email after applying the changes.

See Also

Keywords

smtp 5.7.26, email bounce, unauthenticated sender, spf fail, dkim fail, dmarc alignment, relaxed alignment, outbound email, return-path, mail-from, mta configuration, unauthorized sending ip, receiving domain restrictions, mx txt records, email deliverability , KBA , CEC-CRM-EML , Emails for SAP Sales/Service Cloud , Problem

Product

SAP Service Cloud Version 2 all versions