Sending & delivery events
Throttling (rate limiting)
When a mailbox provider temporarily limits how much mail it accepts from you, usually answering with a temporary error, because your volume jumped suddenly or your reputation is uncertain.
What throttling looks like
The receiving server answers with a temporary SMTP error, such as a 421 or 450 code, often with a message like "try again later". The mail isn’t lost. It stays in the sending queue and is retried, so it arrives late rather than not at all.
Why providers throttle
- A new domain or IP sending more than its history justifies.
- A sudden spike in volume compared with your normal pattern.
- Rising complaints or bounces.
- Too many simultaneous connections from one sender.
How to avoid it
Warm up new domains gradually, keep your sending pattern steady, and spread large campaigns over time rather than sending everything at once. Persistent throttling is an early warning about reputation, so check your complaint and bounce rates when you see it.
Let MailCraft handle the details
Authentication, bounces, complaints and suppression are built in. Start free with 3,000 emails a month, no credit card required.
Start freeRelated terms
Full glossarySending & delivery events
Soft bounce
A temporary delivery failure — a full mailbox or a momentarily unreachable server. Soft bounces are worth retrying a few times before giving up.
Deliverability
Domain warm-up
Gradually increasing how much email you send from a new or newly used domain, so mailbox providers can build a reputation for it before you send at full volume.
Deliverability
IP warm-up
The practice of gradually increasing send volume from a new domain or IP so mailbox providers build trust in your sending pattern before you reach full volume.