A retail client's flash-sale email was their best-performing campaign of the quarter. 500,000 customers, 38% open rate, 12% click rate. First time they ran it, their website's checkout service crashed.
Half a million emails landed in inboxes within 4 minutes. Fifty thousand customers clicked the link within the next two. The checkout service hadn't been sized for that kind of traffic spike. Conversion crashed with the checkout.
The IT team asked for "some way to stop the email from creating these spikes." SFMC Send Throttling is exactly that.
How Send Throttling works
In the Send Definition (or Guided Send), set a per-hour send rate. SFMC will pace the send at that rate instead of dispatching everything at once.
Total audience: 500,000
Throttle: 50,000 per hour
Send duration: 10 hours
Same 500k customers receive the email, but spread across 10 hours instead of 4 minutes. The website traffic is smoothed, the checkout service handles it, and the conversion rate actually comes in higher because nothing's breaking.
Where to configure
- Guided Send wizard (Email Studio > Interactions > Send)
- Send Definition inside an Automation or Journey (Send Email Activity settings)
The UI lets you specify either:
- Maximum emails per hour (explicit rate), or
- Total time window (SFMC computes the rate)
Both produce the same result; pick whichever the stakeholder is more comfortable thinking about.
Use cases beyond flash sales
Flash sales, limited-stock promotions
Covered above. Protects the checkout and inventory systems from their own marketing success.
Time-zone respect
Send scheduled for 9am local time, but your subscribers span 4 time zones. Without throttling, everyone receives the email at once - which means 9am for some, 6am for others, noon for others. Throttling combined with per-subscriber timezone data can approximate a "9am their time" delivery.
IP warming
Covered in the warming post. When ramping a new IP, throttling spreads the daily allocation across hours instead of dumping it in the first minute. Looks natural to ISPs.
Reducing reply volume spikes
Some automated replies generate customer service load (people hitting "reply" to newsletters even though the From is no-reply). Throttling spreads those replies over more hours, giving the CS team time to triage.
Capacity-constrained downstream systems
Any downstream system that processes a spike poorly benefits from throttled upstream sends. CRM sync, loyalty point crediting, chatbot handoffs - all can get overwhelmed by 500k simultaneous triggers.
Side effects to consider
Open rate distribution changes
Subscribers opening early in the throttled window see the email fresh; subscribers opening late in the window see an email that's been waiting in their inbox for hours. Competition from subsequent emails in the inbox can reduce open rate for the later subscribers.
Trade-off: smoother downstream load vs. potentially lower open rate on the tail end. For flash sales where the offer is time-boxed, a throttled send is usually still the right call because the alternative is a broken checkout.
Latency for transactional
Don't throttle transactional emails (password reset, OTP, order confirmation). Those need to arrive immediately. Throttling them introduces delay customers notice and complain about.
Delay in closing the campaign
A 10-hour send won't finish at 9am. If you need the full send completed by a specific time, backward-calculate: start time minus total-window = required launch time.
Typical throttle rates
Per-hour rates we use on different send types:
- Flash sale / high-CTR campaign: 25k-50k/hour (protects web infra)
- Newsletter: usually no throttle (engagement is spread naturally)
- IP warming: 500-2,000/hour on the new IP during week 1-2
- Low-risk promotional: 100k+/hour if infra can handle it
Start conservative on a new campaign type and loosen once you have evidence the downstream can absorb the volume.
Takeaway
Send Throttling is an often-skipped feature that prevents an entirely self-inflicted class of outage. Any send that drives concentrated downstream traffic - flash sales, stock launches, checkout-heavy promotions - should have throttling configured. 5 minutes of setup saves the day the email over-performs.
Sizing an SFMC campaign with downstream infra limits? Our Salesforce team configures throttling, schedule windows, and infrastructure coordination on production engagements. Get in touch ->
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