CVE-2026-67447: Mailpit: SMTP DATA line reader buffers over-limit input before size enforcement

Mailpit’s SMTP DATA reader enforces the configured MaxMessageSize only after bufio.Reader.ReadBytes('\n') has already buffered a complete DATA line. A remote unauthenticated SMTP client can send one line larger than the configured message-size cap and force memory allocation before Mailpit returns the expected 552 5.3.4 rejection, leaving patched versions still exposed to a single-line incomplete-fix variant of the earlier SMTP DATA body-size issue.