# Multi-Server Rotation in AcelleMail — How the Dispatcher Picks

> Once you have 2+ sending servers, the AcelleMail dispatcher distributes per-message across them. This guide explains the rotation logic, how to verify it in the admin pool, and how to override for deterministic routing.

- Canonical URL: https://acellesend.com/kb/articles/multi-server-rotation-pattern
- Author: Lena Hart, Email Deliverability Editor
- Category: Sending & Deliverability — SMTP setup, bounce handling, sender reputation, IP warmup, throttling
- Content type: tutorial · Difficulty: intermediate
- Tags: Acellemail
- Published: 2026-07-04 · Last updated: 2026-08-15
- Reading time: 6 min
- Publisher: AcelleMail Knowledge Base, https://acellesend.com/kb

---

## How the dispatcher picks a server per message

When you launch a campaign with multiple servers in your plan's pool, AcelleMail's send-pipeline picks one server per message. The pick order:

1. **Filter by plan** — only servers attached to the campaign owner's plan are candidates
2. **Filter by quota** — servers that hit their daily/hourly cap drop out for this window
3. **Filter by warmup state** — servers in active warmup get priority for low-volume sends and are skipped for high-volume burst sends
4. **Round-robin within remaining candidates** — even distribution; no stickiness per recipient or per campaign

The pick happens at job-handler time (not at campaign-launch time). Mid-send, if one server hits quota or fails its health check, subsequent messages skip it automatically — no manual failover.

## Verify the rotation in the admin pool
### Open the sending-server detail

In AcelleMail's sidebar, click **Sending → Sending servers**. The list shows every server connected to this account with its status chip, sending limit, and last activity:

![Customer sending-server list](https://acellesend.com/plugins/acelle/landing/images/help/kb/sending-servers/01-customer-server-list.png)

Click into the row you want to configure. The detail page surfaces Connection settings (host / credentials), Configuration (server name, default from, sending limit, bounce + FBL handler), and the **Test connection** / **Send test email** buttons in the toolbar:

![Server detail — Connection + Configuration](https://acellesend.com/plugins/acelle/landing/images/help/kb/sending-servers/02-server-detail-full.png)

For pool-wide visibility, open the admin view:

![Admin pool — total / active / inactive + per-type breakdown](https://acellesend.com/plugins/acelle/landing/images/help/kb/sending-servers/08-admin-pool-list.png)

The "Server types" bar shows the composition of your pool. The per-server row shows daily / hourly usage. After launching a campaign, refresh every few seconds — usage counters increment across multiple servers, confirming rotation is active.

## Deterministic routing — when you need it

Round-robin is the right default. Some scenarios want deterministic routing:

| Scenario | Solution |
|---|---|
| Marketing on IP A, transactional on IP B | Two Plans: "Marketing" attaches Server A only; "Transactional" attaches Server B only. Assign customers/campaigns to the right plan. |
| EU customers sent from EU server (GDPR data residency) | Per-customer Plan assignment based on region tag |
| New IP being warmed — start at 10% traffic | Per-server warmup config sets daily cap that grows automatically |
| Campaign with extra-strict deliverability requirements | Attach to a Plan whose pool is a single high-reputation server |

### Setting it up via Sending Plans

In the admin sidebar **Plans → [plan] → Sending Servers**, attach the specific servers you want this plan's customers to use. Customers on this plan only see those servers; the dispatcher picks among them.

The admin server detail shows which plans each server is attached to:

![Admin server detail — plan attachments visible](https://acellesend.com/plugins/acelle/landing/images/help/kb/sending-servers/09-admin-server-detail.png)

## Common UI signals + fixes

| Symptom | Likely cause | UI fix |
|---|---|---|
| All campaigns hit Server A, B sits idle | Server B not attached to the plan, OR Server B's daily quota set to 0 | Admin → Plan → Sending Servers (attach); OR Server B → Sending limit (raise from 0) |
| Server A usage hits 100% daily quota at noon | Pool too small for daily volume | Raise per-server quotas OR add a third server |
| Rotation seems uneven (60/40 split) | One server is warming (lower quota) | Expected — the dispatcher avoids overloading the warming server |
| Mid-campaign one server flatlines | Health check failing — connection refused or auth error | Open that server's detail → Test connection → fix the credentials/network |
| Campaign log shows "Used Server X" for ALL messages | Single-server plan attached, OR all-but-one server disabled | Confirm via Admin → Plan; enable additional servers in plan attachment |

## Distribution patterns over time

Healthy multi-server distribution over a 30-day window should look like:

| Pattern | What it means |
|---|---|
| 33/33/34 across 3 servers | Healthy round-robin, all servers similar quota + warmup state |
| 80/15/5 across 3 servers | One large + two warming. Expected during ramp-up. |
| 60/40 across 2 servers | Server B has a lower daily cap or recently warmed up. Adjust caps to align. |
| 100/0 across 2 servers | Server B disabled or out of the plan. Bug — fix. |

<details>
<summary><strong>Advanced: weighted rotation, sticky routing, and quota-orchestration patterns</strong></summary>

AcelleMail's default round-robin is uniform. For non-uniform distribution patterns, the lever is per-server sending limit + how plans attach:

**Weighted rotation via quota** — to send 70% on Server A and 30% on Server B, set:

```
Server A sending_limit: 70,000/day
Server B sending_limit: 30,000/day
```

Pool dispatcher continues round-robin but Server B caps at 30k, after which only A receives more. Approximates a 70/30 weight at high volumes; can drift at low volumes due to round-robin.

**Sticky routing per recipient** — out of the box AcelleMail does NOT stick a recipient to a specific server. For sender-domain consistency (some receivers warm to a specific IP), the workaround is one-IP-per-tenant: create a separate Plan per tenant, each with a dedicated server. Operationally heavy; only worth it for high-stakes tenants.

**Quota orchestration across vendors** — for very high volume, the typical pattern is:

```
Tier 1 (highest reputation, lowest cost):  Amazon SES — 60% of volume
Tier 2 (mid reputation, growing):          Mailgun  — 25%
Tier 3 (newest IP, in warmup):             Postmark — 15%
```

Adjust quotas weekly as warmup-tier servers earn reputation. Auto-grow Postmark from 15% → 25% as bounce rate stays clean.

**Automated rebalancing** via API:

```bash
# Daily script: read per-server bounce rate, raise quota on clean + lower on dirty
for server_uid in $SERVER_UIDS; do
  br=$(curl -sH "Authorization: Bearer $ADMIN_TOKEN" \
    "https://acellemail.com/api/v1/admin/sending-servers/${server_uid}/stats" \
    | jq '.bounce_rate_last_24h')

  current=$(curl -sH "Authorization: Bearer $ADMIN_TOKEN" \
    "https://acellemail.com/api/v1/admin/sending-servers/${server_uid}" \
    | jq '.sending_limit')

  # If bounce rate < 1%, raise quota 10%; if > 3%, cut quota 25%
  new=$(python -c "
  br=$br; c=$current
  print(int(c*1.1) if br<0.01 else int(c*0.75) if br>0.03 else c)
  ")

  curl -X PATCH -H "Authorization: Bearer $ADMIN_TOKEN" \
    -d "{\"sending_limit\": $new}" \
    "https://acellemail.com/api/v1/admin/sending-servers/${server_uid}"
done
```

Run as a daily cron. Self-tuning pool that prefers clean servers, throttles dirty ones automatically.

**Failure-mode rotation** — when a server fails mid-send, AcelleMail's dispatcher marks it `degraded` and skips it for retries within the same campaign run. Re-evaluates per-server every 5 minutes via health-check ping. Tune the health-check interval in `config/sending-server.php` if you need faster failover (cost: more probe traffic).

</details>

## Related articles

- [Configuring multiple sending servers](https://acellesend.com/kb/articles/configuring-multiple-sending-servers)
- [Sender reputation monitoring](https://acellesend.com/kb/articles/sender-reputation-monitoring)
- [Sending throttling strategies](https://acellesend.com/kb/articles/sending-throttling-strategies)
- [IP warmup schedule for new sending servers](https://acellesend.com/kb/articles/ip-warmup-schedule-for-new-sending-servers)

---

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