After the Jun 2026 no-DKIM incident (campaign mail went out unsigned ->
junked/blocked, ~23% delivery), DKIM is fixed and we must re-send to the
now-signed audience. The builder previously held Google AND Microsoft AND
consumer-MX out until warmup day 30; that blocks the re-send of the Microsoft-
hosted business domains that are most of the list.
Add MAIN_EXCLUDE_OPERATORS (comma-separated mx_provider labels) to override
WARMUP_EXCLUDE_OPERATORS. Set it to 'google' in the workers env so we send to
everything EXCEPT Google's consumer inboxes (still recovering reputation),
including Microsoft/Hotmail. Drives both the SQL exclude and the per-operator
daily cap consistently. Unset => prior default; '' => exclude nobody.
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