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Administrators - How to fix "450 4.7.1 [TMD] Rate limiting for excessive data use"

In its Sender Best Common Practices document, the Messaging, Malware, Mobile Anti-Abuse Working Group (M3AAWG) writes:

"[Email sending services] that send large volumes of email on behalf of their clients are at the mercy of their worst clients' worst practices. All ESPs must have some type of pre-send vetting process to proactively identify malicious senders before they mail and they must all have a post-send vetting process to monitor clients after they mail."

The "too-much-data," or "TMD" rate-limiting policy described in this article is just one of many automated policy actions implemented by MailChannels to ensure the continuous availability and reliability of our email delivery platform for millions of endusers. In this article, we describe why the TMD rate limit exists, how it works, and what you can do if this rate limit has caused messages from your domain to be temporarily rejected by our service.

What is TMD Rate Limiting?

TMD (Too Much Data) rate limiting is an adaptive policy mechanism designed to protect MailChannels' infrastructure and reputation, to reduce abuse, and to maintain high deliverability rates for all customers and endusers. The TMD policy targets less than 0.01% of sending domains and is crucial in preventing spam waves that could compromise our service quality and imperil the excellent reputation we maintain with the internet community.

How Does TMD Rate Limiting Work?

The TMD rate limit is based on the total amount of email message body data sent by a domain within sliding time windows of varying lengths. These windows and limits are adjusted periodically by our team in response to the recent behaviour of abusive senders. For security reasons, we do not publish the specific limits or window durations.

When a domain exceeds any of the TMD thresholds, we may respond with a temporary TMD error such as, "450 4.7.1 [TMD] Rate limiting for excessive data use," for so long as one of the TMD rate limits has been exceeded. The TMD rate limit policy is temporary and automatically resolves as soon as enough time has passed for the domain's total email data volume to fall beneath all of the thresholds of the TMD policy.

Although we recognize that any rejection - even a temporary rejection - is frustrating for senders, temporarily rate limiting a sender to prevent abuse aligns with M3AAWG's best practices:

"Temporary failures indicate that the sending server should re-queue the message and try again later. A common temporary failure may be that the receiving server is too busy with other incoming email and does not have sufficient resources to accept a given message or messages. Another cause may be that the receiver has detected unusual mailing characteristics from an IP address and has decided to stop accepting email from it for a set length of time."

Why is TMD Rate Limiting Necessary?

  1. Infrastructure Protection: While MailChannels maintains an elastic infrastructure within Amazon Web Services and has ample ability to scale up to handle surges of traffic, significant surges in data volume from even a single domain name can temporarily cause instability in our service while it scales. As with any large scale internet service, targeting individual users with a temporary limit is always preferable to risking instability that might affect all users.
  2. Spam Prevention: The TMD rate limit helps block spammers who often use multiple accounts to send high volumes of email suddenly. Spammers can set up accounts across numerous MailChannels customers and coordinate these accounts to send a surge of spam emails in a very short timeframe. Although our automated email content filtering systems are fast to respond to new spam campaigns, by the time we have responded to such a surge, email receivers may already be rolling out a blocking response that would hinder our ability to deliver email for other customers. TMD gives our systems time to respond to signals before it's too late.
  3. Reputation Management: TMD is just one of many policies in place to maintain MailChannels' excellent email reputation, ensuring reliable delivery for all customers. MailChannels is one of the most highly regarded email sending services on the internet; this reputation benefits the vast majority of our customers and their endusers, at the cost of inconveniencing a very small fraction of senders.

What Happens When TMD Rate Limiting is Triggered?

  1. Temporary Rejections: TMD errors are temporary (4xx status codes) and resolve automatically when the limits that triggered the rejection are no longer occurring. Continued connection attempts during a period when TMD is in place for a sender domain will not contribute to lengthening the rate limited period.
  2. No Billing Impact: As is the case with all temporary SMTP rejections, we do not bill customers for messages that receive TMD rejections. Our billing system only counts messages that are either fully received (i.e. "250 Ok") or those that result in a permanent rejection (i.e. 500-599).

Options for High-Volume Senders

If you're a legitimate high-volume sender encountering TMD rate limiting or if your customer is such a sender, you have two main options:

  1. Adjust Sending Rate: Reduce your domain's sending rate or the average size of your messages. Message size can be reduced considerably by moving images and attachments to a CDN rather than including them in the message body. We often find the TMD-affected senders have a message size that is up to 15,000-times higher than median messages; this message size problem often stems from naivety rather than being intentional. Simply moving attachments and images to an external URL usually resolves TMD and may improve deliverability.
  2. Request a Review: You may contact the MailChannels support team to arrange a review of a  high volume sending domain. In limited cases, we will make an exemption for domains that need to send large amounts of data that is reliably legitimate in nature. As per our terms of service, all policy decisions made by the MailChannels team in relation to rate limiting to control abuse are final. Mail filtering is adversarial in nature and this stipulation must remain in place in order for MailChannels to continue offering reliable email delivery for all customers and endusers.

Upcoming Features for High-Volume Senders

We're developing new services aimed at high-volume senders, which will provide:

  • Better tools for understanding email delivery
  • Improved deliverability features
  • Potential revenue opportunities for resellers

Stay tuned for announcements about these new offerings.

Best Practices to Avoid TMD Rate Limiting

  1. Spread out large email campaigns over time rather than sending all at once.
  2. Ensure you have proper consent from all recipients.
  3. Regularly clean your email lists to remove inactive or invalid addresses.
  4. Monitor your sending patterns and adjust if you're approaching the rate limits.

Contact Us

If you have any questions about TMD rate limiting or need assistance with high-volume sending, please don't hesitate to contact our support team. We're here to help you achieve the best possible email delivery outcomes while maintaining the integrity of our service.

Remember, our goal is to provide reliable, high-quality email delivery for all our customers. By working together to maintain good sending practices, we can ensure the best possible delivery rates for everyone using MailChannels.

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