TL;DR: One SendGrid account means one reputation pool. When marketing blasts drag transactional mail into spam, create subusers for each stream (transactional, marketing), give each its own API keys and, at volume, its own IPs or IP pools, authenticate the sending domains per subuser, and monitor stats per subuser instead of account-wide. After that, a complaint spike in marketing stops moving transactional inbox placement.

What it looks like:

```text
Symptom: password resets and receipts land in spam right after a big marketing
send, on the same account.
```

Steps:

1. Confirm the streams are coupled. Compare engagement and spam-report rates per mail stream. Success: marketing's numbers are bad and transactional delivery sagged at the same time.
2. Create subusers for transactional and marketing, each with its own API keys. Success: each stream sends on separate credentials.
3. Authenticate the sending domains per subuser so the identities stay separate. At volume, give each stream its own dedicated IPs or IP pools. Success: per-subuser stats show distinct sending identities.
4. Monitor stats per subuser, not just account-wide, so a problem in one stream does not hide in the average. Success: a marketing complaint spike no longer moves transactional inbox placement, visible in per-subuser stats.

When to use: transactional and marketing share a SendGrid account and one's behavior is dragging the other down.

When not to use: a single mail stream (nothing to isolate), unauthenticated sending domains, or bad list quality (fix those first, subusers wont save you).

Compatibility: SendGrid subusers on any plan that supports them. Event Webhook stats are subuser-scoped.

Variants:
- sendgrid marketing hurting transactional deliverability
- split sendgrid reputation streams

Root cause: reputation aggregates per sending identity and IP. Shared identity means shared fate, so one high-complaint blast poisons the engagement signals for the transactional mail sharing it.

Edge cases:
- At low volume, a dedicated IP can hurt more than help (no warmup). Only split IPs when you have the volume to warm them.
- Subusers split the stream, not the domain reputation. If both streams share a domain with no per-subuser auth, isolation is weaker.

---

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## Selected public attribution and original notices/references

```json
{
  "author": "issueatlas",
  "client": null,
  "notices_and_references": [
    "https://docs.sendgrid.com/for-developers/tracking-events/event",
    "https://docs.sendgrid.com/ui/account-and-settings/how-to-set-up-reverse-dns"
  ]
}
```