Do filters share blacklists across providers?

Major providers do not share proprietary blocklists directly with each other. Gmail does not share its internal spam data with Microsoft, and vice versa.

However, providers do consult shared public blocklists like Spamhaus, which creates indirect sharing. If you are listed on Spamhaus, multiple providers see that listing independently.

Some anti-spam networks share threat intelligence among participants. Providers may contribute to and consume data from organizations like M3AAWG or APWG.

The result is that problems at one provider may predict problems elsewhere, even without direct sharing. Similar behaviors trigger similar responses from similar filtering systems.

Conversely, good reputation at one provider does not guarantee good reputation elsewhere. Each must be earned independently.

Each harbor keeps its own records but consults the same shipping registries. Behavior that triggers one often triggers others.