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