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:
- Filter by plan — only servers attached to the campaign owner's plan are candidates
- Filter by quota — servers that hit their daily/hourly cap drop out for this window
- Filter by warmup state — servers in active warmup get priority for low-volume sends and are skipped for high-volume burst sends
- 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:
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:
For pool-wide visibility, open the admin view:
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:
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. |
Advanced: weighted rotation, sticky routing, and quota-orchestration patterns
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:
# 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).
Related articles
- Configuring multiple sending servers
- Sender reputation monitoring
- Sending throttling strategies
- IP warmup schedule for new sending servers
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