Definition
Email concurrent connections are the number of simultaneous SMTP sessions a sending server maintains with a receiving server at any given moment. Each connection can carry multiple messages in sequence, but the receiving server caps how many connections it will accept from a single sender. Managing this number is a key part of configuring sending infrastructure for both throughput and good behaviour.
How It Works
When a sender transmits a campaign, it opens SMTP connections to the recipient's mail server. The sending platform's settings determine how many connections are opened in parallel and how many messages flow through each one. The receiving server advertises or enforces its own limits and will reject additional connection attempts with a temporary failure once those limits are reached.
The two related settings that matter are:
| Setting | What it controls |
|---|---|
| Concurrent connections | How many simultaneous sessions are open at once |
| Messages per connection | How many messages are sent over a single session before it closes |
Higher concurrency increases throughput up to a point, but opening too many connections looks abusive and can trigger throttling or blocking.
Why It Matters
Receiving providers watch connection behaviour closely. A sender that opens an excessive number of simultaneous connections, or opens and closes connections rapidly, resembles a spam bot. Well-behaved senders open a moderate number of connections and reuse each one for several messages, which is more efficient and signals legitimate infrastructure.
Best practices for concurrent connections include:
- Start with conservative defaults, often around 10 to 20 connections per receiving domain for mid-sized senders.
- Reuse connections for multiple messages rather than opening a new connection per message.
- Increase concurrency only when a provider is accepting mail quickly and deferrals are low.
- Keep connection counts stable rather than spiking unpredictably.
These settings interact with email throttling, and both should be tuned together during email warmup to avoid overwhelming receiving servers.
Example
A SaaS company migrates to a new sending platform and notices deferrals at a major provider. Its platform was configured for 100 concurrent connections, far more than the provider tolerates. The team reduces the setting to 15 and spreads messages across connections; deferrals disappear and throughput remains adequate.
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Frequently Asked Questions
There is no single correct number, but a moderate setting in the range of 10 to 20 connections per receiving domain is a common starting point for mid-sized senders. The right value depends on volume, provider limits, and observed deferrals.
Only up to a point. Beyond the receiving server's tolerance, additional connections produce temporary failures rather than faster delivery. Throughput is ultimately capped by the provider, not just the sender's settings.
Yes. Excessive or erratic connection behaviour is a known signal of abusive sending and can contribute to throttling or a lower sender reputation. Conservative, consistent connection patterns are safer.
No. Concurrent connections describe how many simultaneous sessions are open, while throttling describes rate limits the provider applies. The two are related, because connection behaviour can trigger throttle responses.