Throttling happens when a mailbox provider deliberately limits how fast it accepts mail from you. Instead of accepting every message, it responds to some with a temporary refusal, usually an SMTP code starting with 4, meaning “try again later”. The sending server waits and retries. Mail eventually gets through, but slowly, and some may expire undelivered if throttling lasts too long.
Providers throttle to protect their users and systems. The usual triggers are a sudden increase in volume, sending from a new IP or domain without warm-up, a drop in reputation after complaints or bounces, or too many simultaneous connections from your servers. It is the provider saying “we aren’t sure about you yet” rather than rejecting you outright.
Sending platforms handle throttling by slowing down for the affected provider, spreading messages over time and retrying deferred mail with increasing gaps. Most of this happens automatically. What you see is slower delivery, more soft bounces at one provider, or a campaign that takes hours rather than minutes to finish.
To avoid it, increase volume gradually rather than in leaps, especially before festive seasons when many senders spike at once; warm up new IPs and domains; send to engaged contacts first in large campaigns; and keep complaint rates low. If throttling continues for days, treat it as a reputation warning and review recent list sources and complaint rates.
A Surat textile wholesaler usually sends 5,000 emails a week, then sends 60,000 on the first day of a Diwali sale. Gmail defers a large share, and the campaign takes nine hours to deliver. The next year it builds up volume over three weeks before Diwali and sends the big campaign in waves.
When you send through your own server, MailNesto lets you set a daily sending cap and a warm-up ramp, so volume rises gradually instead of in sudden jumps.