Flash sale email to 500k customers lands in inboxes in four minutes. Traffic hits the website at once, checkout crashes, the conversion spike turns into downtime. Send Throttling spaces the send.
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 four 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.
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.
Same 500k customers receive the email, but spread across ten hours instead of four minutes. The website traffic is smoothed, the checkout service handles it, and the conversion rate actually comes in higher because nothing's breaking.
The UI lets you specify either:
Both produce the same result; pick whichever the stakeholder is more comfortable thinking about.
Covered above. Protects the checkout and inventory systems from their own marketing success.
Send scheduled for 9am local time, but your subscribers span four 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.
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.
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.
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.
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.
Don't throttle transactional emails (password reset, OTP, order confirmation). Those need to arrive immediately. Throttling them introduces delay customers notice and complain about.
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.
Per-hour rates we use on different send types:
Start conservative on a new campaign type and loosen once you have evidence the downstream can absorb the volume.
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. Five 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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