# X_MAIL_HEADER_INVALID

## Means

a header value carries a line break

## Typical cause

interpolated data with a CR/LF reached `Subject` — header injection. `meta.reason: 'non-ascii'` is the other case: a non-ASCII mailbox in `From`/`To`/`Cc`/`Reply-To` reached the SMTP message builder

## Fix

reject the value at its own boundary — `t.string().pattern(/^[^\r\n]*$/)` on the field that feeds the header — then re-send. Never strip the break: a silently rewritten header hides the injection attempt

[Mail](/error-codes#mail)