SMTP Load Balancing
When you have multiple SMTP connections configured, Score CRM offers several strategies to distribute your email volume across them.
Load Balancing Modes
Single
Use exactly one SMTP connection for the entire campaign. This is the simplest option and works well when:
- You only have one SMTP provider
- You want all emails to go through a specific connection
- You're warming up a new IP/domain
Percentage Split
Assign a specific percentage of traffic to each connection. The percentages must add up to 100%.
Example:
| Connection | Percentage |
|---|---|
| SendGrid (Production) | 70% |
| SparkPost (Secondary) | 30% |
This mode is useful for:
- Gradually shifting traffic between providers
- IP warm-up (slowly increasing a new connection's share)
- A/B testing SMTP providers for deliverability
- Risk mitigation (don't put all eggs in one basket)
With percentage split, SMTP connections are pre-assigned to recipients at campaign start time. This ensures exact percentage distribution and avoids runtime routing overhead.
Round Robin
Alternates between connections evenly. Each batch of emails rotates to the next connection in the list.
This mode is useful for:
- Even distribution across providers
- Maximizing overall throughput
- Balancing rate limits across multiple IPs
How Pre-Assignment Works
For Percentage Split mode, Score CRM assigns each recipient an SMTP connection during the recipient resolution phase (before any emails are sent):
- Campaign launches → recipient list is resolved
- Each recipient is assigned an SMTP connection based on the configured percentages
- Emails are sent through the pre-assigned connection
- No runtime routing decisions needed
This approach ensures:
- Exact percentage distribution
- No hot-spots or uneven loading
- Deterministic behavior (same recipient always uses the same connection if the campaign is paused and resumed)
Choosing a Strategy
| Scenario | Recommended Mode |
|---|---|
| Single provider | Single |
| Multiple providers, specific ratio | Percentage Split |
| Multiple providers, equal distribution | Round Robin |
| IP warm-up alongside production | Percentage Split (start at 10%, increase gradually) |
| Maximum throughput | Round Robin |
Combining with Throttling
Load balancing works alongside throttling. If you set a campaign-level throttle of 6,000 emails/hour with a 70/30 percentage split:
- Connection A sends ~4,200 emails/hour (70%)
- Connection B sends ~1,800 emails/hour (30%)
Each connection's individual rate limits are also respected.