Email Encyclopedia: What is Send Rate Limiting

Table of Contents

Send Rate Limiting refers to a technical mechanism in electronic mail systems that limits the number or frequency of electronic mails allowed to be sent within a unit of time to prevent email abuse, spam proliferation, and server overload issues. It is one of the security and management strategies widely adopted by email servers and email service providers (ESPs).

Send rate limiting is typically implemented by mail server software (such as Postfix, Sendmail, Microsoft Exchange, etc.) or cloud email service platforms (such as Amazon SES, SendGrid, Mailgun, etc.), aiming to balance system resource usage, ensure quality of service (QoS), and maintain the healthy operation of the internet email ecosystem.

Background and Necessity #

As electronic mail has become an important tool for modern communication, issues of its abuse have also become increasingly serious. For example:

Spam: Large amounts of advertisements, scams, and malicious link emails sent without user permission.
Email bombing: Attackers attempting to paralyze target mailboxes or servers by sending a large number of emails in a short time.
Abuse after account compromise: Hackers using compromised user accounts to mass-send spam.

These problems not only affect the recipient’s experience but can also cause excessive load on mail servers and even trigger blacklist banning measures by anti-spam organizations, affecting the reputation of the entire domain or IP address.

Therefore, send rate limiting has emerged as an effective defense mechanism.

Principles and Working Mechanisms #

The core idea of send rate limiting is to control email traffic by setting upper limits on email sending behavior. Common limitation methods include:

1. Time Window Limitations #

Limiting the total number of emails that an account or IP address can send within a fixed time period (such as per hour, per day). For example:

Maximum of 200 emails per hour
Maximum of 500 emails per day

This limitation applies to most commercial email platforms, such as Gmail, Outlook, etc.

2. Connection Number Limitations #

Limiting the number of SMTP connections that the same client or IP address can establish simultaneously. This can prevent malicious programs from using multi-threading to send a large number of emails concurrently.

3. Burst Traffic Control #

Allowing email sending to exceed the average rate for a short time, but still requiring compliance with long-term rate limits overall. For example:

Average of 5 emails per second
Can reach up to 20 emails per second during burst periods, but the duration cannot exceed 10 seconds

This mechanism is commonly used for business customers who need to send batch emails, ensuring efficiency without impacting the server.

4. Identity-Based Limitations #

Dynamically adjusting sending quotas based on user identity (such as administrators, regular users), account type (free/paid), historical behavior, and other factors. For example:

Free accounts limited to 100 emails per day
Paid accounts limited to 1000 emails per day
High-risk accounts automatically have reduced quotas

Implementation Methods #

Different email systems and service platforms adopt different technical means to implement send rate limiting. The following are several common methods:

1. Queue Management #

Mail servers place emails to be sent in a queue and send them one by one according to rate limiting rules. This method can effectively control concurrency and overall traffic.

2. Token Bucket Algorithm #

This is a classic traffic control algorithm. The system adds tokens to a “bucket” at a fixed rate, with each email sent consuming one token. If there are no tokens in the bucket, emails wait in the queue.

3. Leaky Bucket Algorithm #

Similar to the token bucket, but it “leaks” requests at a constant rate, with the output rate remaining unchanged regardless of when or how many requests arrive. This is suitable for handling uniform traffic.

4. Database Recording and Analysis #

Some advanced systems combine database records of user sending behavior to dynamically assess whether it belongs to an abnormal pattern. For example, if an account suddenly sends thousands of emails within a few minutes, the system will temporarily restrict its sending permissions.

Applicability to Different Users #

Send rate limiting applies to various types of users and scenarios:

1. Individual Users #

Most email service providers set default sending limits for personal accounts to prevent accounts from being used to send spam after being compromised. For example:

Gmail: About 100 per hour, about 500 per day
Outlook: About 300 per day

2. Business Users #

Enterprise email systems typically allow higher sending quotas and support customized rate limiting policies. For example:

Self-hosted mail servers can configure Postfix’s smtpd_client_message_rate_limit
Using third-party email services (such as Amazon SES) allows dynamic quota adjustment through APIs

3. Marketing Email Senders #

Businesses engaged in large-scale marketing emails typically need to apply for higher quotas or use dedicated email services. These services provide detailed statistical reports and flexible rate control options.

Impact of Send Rate Limiting #

Positive Impact #

Reducing Spam Propagation: Limiting abnormal sending behavior effectively curbs the spread of spam.
Protecting Server Stability: Avoiding server crashes or slow responses due to sudden traffic surges.
Improving Email Delivery Rate: Reasonably controlling the sending rhythm helps increase the probability of emails reaching the inbox.
Enhancing Account Security: Preventing large-scale email attacks after account compromise.

Potential Issues #

Delayed Legitimate Email Delivery: If rate limits are too strict, normal emails may not be delivered promptly.
Decreased User Experience: Business users may have operations interrupted due to insufficient quotas when sending large batches of emails.
Risk of Misjudgment: Some normal behaviors may be identified as abnormal, leading to erroneous restrictions.

How to Address Send Rate Limiting #

For users or developers who wish to bypass or optimize send rate limits, here are some suggestions:

1. Reasonably Plan Email Sending Schedule #

Send emails in batches, avoiding concentrated peak periods
Set up appropriate retry mechanisms, pausing and retrying later when hitting limits

2. Apply for Increased Quota #

Apply to the email service provider to remove or increase default limits
Provide real business information and domain verification to enhance trust

3. Use Professional Email Services #

Utilize third-party email platforms such as SendGrid, Mailgun, Amazon SES for greater flexibility and better delivery rates

4. Monitoring and Log Analysis #

Regularly check email sending logs to identify the causes of limitations
Use monitoring tools to keep track of sending status and quota usage in real-time

SMTP (Simple Mail Transfer Protocol): The basic protocol for sending electronic mail.
DNSBL (DNS-based Blackhole List): A DNS-based blacklist used to block known spam sources.
SPF (Sender Policy Framework): An email authentication mechanism that prevents forged sender addresses.
DKIM (DomainKeys Identified Mail): An email signing technology used to verify the authenticity of email sources.
DMARC (Domain-based Message Authentication, Reporting & Conformance): An email authentication standard that integrates SPF and DKIM.

Summary #

Send rate limiting is an indispensable security and management mechanism in email systems. It not only helps prevent email abuse and protect server stability but also improves email delivery quality and user experience. Whether for individual users or business organizations, it is important to understand and appropriately address this mechanism when using email services. Through scientific configuration, reasonable scheduling, and choosing suitable service platforms, efficient and stable email communication can be achieved while ensuring security.

References #

Wikipedia: Email spam
Postfix Documentation: Rate control
SendGrid: Sending Limits and Email Sending Policies
Amazon SES Developer Guide: Managing Your Sending Limits
RFC 5321 - Simple Mail Transfer Protocol (SMTP)