Bulk email covers newsletters, promotions, product announcements, and similar sends to a list, as distinct from transactional email triggered by a specific recipient's own action. It generally requires an opt-in consent basis rather than relying on transactional necessity.
Major mailbox providers apply bulk-sender requirements (SPF/DKIM/DMARC alignment, one-click unsubscribe, complaint-rate thresholds) once a sender crosses a daily volume threshold, regardless of whether the content itself is transactional or promotional.
Isolating bulk email onto its own sending lane protects transactional mail from bulk-sending's inherently higher bounce and complaint exposure, since even a well-run marketing program has materially different risk characteristics than a receipt or password reset.