Definition
An email sending cap is a maximum limit on the number of emails that can be dispatched in a defined window — typically per hour or per day. Caps exist at multiple levels: your email service provider enforces account-level limits, dedicated IP addresses have warm-up volume restrictions and receiving mailbox providers like Gmail and Yahoo impose their own rate limits on incoming traffic.
Exceeding a cap typically results in deferred delivery or outright rejection. For businesses running large campaigns, planning around these limits is essential to avoid delivery delays.
Types of Sending Caps
- Platform cap: The maximum volume your ESP allows per hour or day, often tied to your plan tier
- IP warm-up cap: Temporary limits during IP warming, increasing gradually over several weeks
- Provider-specific cap: Limits enforced by Gmail, Yahoo, Outlook on how many messages they accept from a single sender per unit of time
- Authentication cap: Some ESPs limit volume until DKIM and SPF records are fully configured
Best Practices
- Know the caps for every platform tier you operate on
- Plan large sends by staggering across multiple hours to stay within hourly limits
- Never launch a full campaign from a cold IP without a warm-up schedule
- Monitor for throttling signals from specific recipient domains and adjust pacing accordingly
Related Glossary Terms
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.
Email Deliverability
Email deliverability is the ability to land in the recipient's inbox rather than the spam folder. It depends on sender reputation, authentication, list quality, and content practices.
DMARC Alignment
DMARC identifier alignment determines whether the domain in the From header matches the domains used in SPF and DKIM authentication. Strict or relaxed.
Acceptance Rate
Acceptance rate is the percentage of emails submitted for delivery that are accepted by the recipient's mail server, representing the first deliverability checkpoint before inbox placement is determined.
Email Acceptance Rate
Email acceptance rate measures the percentage of emails accepted by recipient mail servers versus total sent. It is a core deliverability metric that reveals whether your sending infrastructure is trusted by receiving servers.