Definition
ISPs impose rate limits to protect their infrastructure and users from spam. When a sender exceeds the limit, the receiving server returns a 4xx SMTP code (typically 450 or 452), telling the sender to try again later. Rate limits are higher for established senders with good reputations and lower for new or unknown senders. Proper IP warmup, consistent sending patterns, and good reputation help increase rate limits over time. If you consistently hit rate limits, you may need to distribute sending across multiple IPs or reduce peak volume.
Related Glossary Terms
AOL Mail for Email Marketers
AOL Mail is a legacy email provider with specific deliverability requirements and rendering quirks, now operating as part of the Yahoo+AOL network under shared infrastructure.
BIMI
BIMI (Brand Indicators for Message Identification) is an email standard that allows brands to display their logo next to their emails in supported email clients. It requires DMARC enforcement at quarantine or reject policy.
Bounce Classification
Bounce classification uses SMTP codes (550, 551, 552, 553, 554, 450, 451, 452) and enhanced status codes to categorise permanent and transient delivery failures.
Bounce Rate Threshold
A bounce rate threshold is the percentage of hard or soft bounces at which email platforms and ISPs begin to penalise sender reputation, typically around 2-5%.
Bounce Rate
Email bounce rate is the percentage of emails that were rejected by the receiving server before reaching the recipient. It is a key indicator of list health and data quality.
Bulk Email Marketing
Bulk email marketing is the practice of sending a single email campaign to a large list of subscribers simultaneously, typically for promotional, newsletter, or announcement purposes.