PreflightSWIFT NACK – Format Error
A NACK indicates that the SWIFT message was rejected at the network or format level before business processing. Format errors prevent the message from being accepted by the network or the next hop.
Last updated: 2026-02-25
Scope: Operational reference. Not scheme legal text. Use scheme documentation for normative rules.
What this reject means
A SWIFT NACK (negative acknowledgment) for format reasons means the message did not meet structural or rule requirements. This is distinct from a business-level reject (e.g. insufficient funds, compliance). Typical causes include invalid or missing BIC, missing or malformed field 59 (beneficiary), incorrect message type or format version, or syntax errors. MT and MX (ISO 20022) have different format rules; the NACK will reference the applicable standard.
When it occurs
Format rejection occurs early in the pipeline: at submission, at the gateway, or at the first processing hop. It happens before settlement or balance checks. The sender receives the NACK and must correct and resubmit. Timing is usually short (same day or within the same cycle) depending on the network and the receiver.
What to check first (triage)
- Validate BIC and beneficiary (field 59) format and presence against the message type and scheme.
- Confirm message type and sub-type match the intended flow (e.g. MT103 vs 103+).
- Check the NACK reason text or code for the exact field or rule that failed.
Prevention signals
- Pre-execution validation of BIC, IBAN (where applicable), and required fields reduces format NACKs before submission.
- Reuse validated message templates and schemas so that structural errors are caught in build or test.
- Monitor NACK rates by message type and corridor to spot systematic format or rule issues.
Related
Preflight models these scenarios deterministically before and after execution.