Start by defining the streams based on purpose.
Transactional mail includes things such as password resets, receipts and account alerts. Promotional/subscription mail includes newsletters, offers and marketing campaigns.
Separate where it helps
Depending on scale and platform, separation can include:
- different From addresses;
- subdomains;
- DKIM selectors;
- provider configurations;
- bounce domains;
- IP pools.
The goal is not to hide poor reputation. It is to make ownership, authentication, monitoring and failures easier to isolate.
Google’s current subscription guidance also recommends separating subscription and non-subscription messages by sending address.