Throttling is the controlled limitation of email dispatch speed — how many emails are sent per unit of time. It operates at multiple levels: MigoSMTP enforces hourly and daily limits (platform throttling), and receiving ISPs independently throttle inbound connections from any single sender (ISP throttling). Understanding both helps you design sending patterns that maximise delivery rates and minimise deferral.
Two Types of Throttling
| Type | Who Enforces It | Effect | How to Handle |
|---|---|---|---|
| Platform throttling | MigoSMTP | Messages queued when hourly or daily limits are reached — never dropped | Plan within your limits; upgrade if consistently exceeded |
| ISP throttling | Gmail, Outlook, Yahoo, and other receiving mail servers | Messages deferred with a 4xx response — MigoSMTP retries automatically | Build sender reputation; send gradually; maintain low complaint rates |
Platform Throttling — How MigoSMTP Enforces Limits
When you submit emails faster than your plan's hourly limit, MigoSMTP accepts all messages into the queue but controls the dispatch rate to stay within your plan's allowed throughput. This works as follows:
- You submit 500 emails via the bulk API at 9:00 AM.
- Your plan allows 150 emails per hour.
- MigoSMTP dispatches the first 150 immediately.
- The remaining 350 are queued and dispatched as follows:
- 10:00 AM reset: next 150 dispatched
- 11:00 AM reset: next 150 dispatched
- 12:00 PM reset: final 50 dispatched
- All 500 messages delivered by approximately 12:05 AM — across ~3 hours, all within limits.
ISP Throttling — How Receiving Mail Servers Limit Your Delivery
Even if MigoSMTP sends within your plan limits, individual ISPs may further throttle how quickly they accept messages from any single IP/domain combination. This is independent of MigoSMTP's limits and is based on your sending reputation with that ISP.
Common ISP Throttling Signals
| SMTP Response Code | Message Example | Meaning |
|---|---|---|
| 421 | Too many connections, please try again later | Server busy — temporary; retry will succeed |
| 450 | Mailbox temporarily unavailable | Soft rejection — retry automatically |
| 451 | Requested action aborted: local error in processing | Temporary server-side error — retry |
| 452 | Insufficient system storage | Mailbox full — soft bounce; retry once, then stop |
| 421 4.7.0 | Try again later, closing connection | Gmail rate limiting — reduce sending rate to this domain |
Factors That Affect ISP Throttling
- IP age and warmup status — new IPs receive stricter throttling until they establish a sending history. A well-warmed IP experiences much less throttling.
- Sender reputation — ISPs track complaint rates, bounce rates, and engagement from your IP/domain. Poor reputation means aggressive throttling.
- Volume spikes — suddenly sending 10x your normal volume triggers ISP spam filters even from established senders.
- List quality — sending to many invalid or unengaged addresses signals to ISPs that you have a poor list, increasing throttling.
- Time of day — ISPs process more connections during their peak hours. Sending during off-peak times for the recipient's timezone can reduce throttling.
How to Reduce ISP Throttling
| Strategy | Effect on Throttling |
|---|---|
| Complete IP warmup before bulk sending | Establishes trust with ISPs; significantly reduces throttling rates |
| Maintain low bounce rate (<2%) | ISPs see a clean, managed list — less reason to throttle |
| Maintain low complaint rate (<0.1%) | Low complaints signal recipient satisfaction — ISPs trust you more |
| Send consistently, not in erratic spikes | Predictable patterns build ISP confidence over time |
| Spread large sends over multiple days | Gradual volume ramp avoids triggering spike-detection algorithms |
| Segment and send to engaged recipients first | High engagement signals early in a campaign warm up the send for the rest of the list |
| Publish DMARC with enforcement | Some ISPs give priority inbox placement to authenticated senders with DMARC p=quarantine or p=reject |
Monitoring Throttling in Your Delivery Reports
Signs of ISP throttling in your MigoSMTP Delivery Reports:
- A significant number of messages with Deferred status — especially to one specific ISP domain (e.g. gmail.com, outlook.com).
- Unusually long delivery times for a campaign — messages that normally deliver in seconds taking hours.
- A pattern of 4xx response codes in the bounce/deferral reason field of your logs.
Filter your Delivery Reports by recipient domain to isolate which ISP is throttling you most severely, and use this information to prioritise list cleaning and warmup efforts for that domain.