What are best practices for setting email domain connection limits for major ISPs?
Michael Ko
Co-founder & CEO, Suped
Published 29 Apr 2025
Updated 19 Aug 2025
5 min read
Managing email domain connection limits for major Internet Service Providers (ISPs) is a crucial aspect of ensuring good email deliverability. These limits, often unstated, can significantly impact how efficiently your emails reach their intended inboxes.
When you send a high volume of emails, ISPs (like Gmail and Outlook) might interpret an excessive number of simultaneous connections or messages per connection as a sign of spamming. This can lead to throttling, where your emails are intentionally delayed, or even outright blocking, which harms your sender reputation.
Understanding ISP connection limits
The primary reason ISPs impose these limits is to protect their users from spam and abuse. By controlling the flow of incoming mail, they can analyze sender behavior, identify suspicious patterns, and maintain the health of their networks. While this helps them, it also means that bulk senders must carefully manage their outgoing email streams.
Without proper configuration, many sending systems default to unlimited connections, which can be detrimental. Attempting to send too many emails too quickly can trigger an ISP's spam filters, leading to a negative impact on your domain and IP reputation. It's often a process of trial and error to find what works best for each provider.
One of the challenges is that most major ISPs do not publicly disclose their exact connection and message limits. These limits are dynamic, often adjusting based on your sender reputation. A good sender with strong engagement rates might experience higher limits than a new sender or one with a poor sending history. This dynamic nature means that a one-size-fits-all approach rarely works.
The risk of ignoring connection limits
Sending emails without considering ISP connection limits can lead to several severe deliverability issues. Without careful management, your legitimate emails might never reach the inbox.
Throttling: Your emails are delivered at a slower pace, impacting timeliness and campaign performance. This is a common way ISPs manage email flow.
Bounce rates: Emails may soft bounce if connection limits are exceeded, requiring re-sending attempts or potentially causing non-delivery.
Reputation damage: Frequent violations can severely harm your sender reputation, making it difficult to reach the inbox in the long term. This impacts overall deliverability.
Factors influencing limits and starting points
Your sender reputation is the primary factor influencing the connection and message limits imposed by ISPs. A strong reputation, built on consistent sending of high-quality, engaged emails, will generally afford you higher limits. Conversely, a poor reputation will lead to more stringent limits and more frequent throttling.
For new IPs or domains, a gradual ramp-up, often called IP warming, is essential. This involves starting with very low volumes and slowly increasing them over days or weeks. This process helps establish a positive sending history with ISPs. Without it, even legitimate emails can be flagged as suspicious due to sudden, high volume.
When starting, or if you're unsure, a common baseline for initial outbound connections is a 10:100 ratio: a maximum of 10 concurrent outbound connections and a maximum of 100 messages per connection. This is a conservative starting point that can then be adjusted. However, it's critical to monitor performance and tweak these numbers based on how different ISPs respond. Some ISPs might still throttle at these levels, while others could handle higher volumes.
New sender setup
IP warming: Begin with small email volumes and gradually increase over time. See our guide on warming email sending IPs.
Conservative limits: Start with low connection and message limits, like the 10:100 ratio, and carefully monitor performance.
Dedicated IPs: Use dedicated IP addresses for consistent reputation building, as shared IPs can be influenced by other senders.
Established sender optimization
Dynamic adjustment: Adjust limits based on real-time feedback from bounce logs and Postmaster Tools from Google and Microsoft.
Continuously monitor and refine your sending strategy for optimal inbox placement. This iterative process allows you to send efficiently without triggering ISP throttling or blocking mechanisms.
Implementing and monitoring connection limits
Effective implementation of connection limits requires a mail transfer agent (MTA) or email service provider (ESP) that allows granular control over sending parameters. You should be able to set limits for concurrent connections, messages per connection, and even messages per hour or day for specific domains.
For self-hosted solutions, configuration files (like those for Postfix or PowerMTA) are where these limits are defined. For ESPs, these settings are typically managed within their platform, though you might have varying degrees of direct control. It's important to understand common limits imposed by providers.
Monitoring is key. Pay close attention to your bounce logs for transient errors or deferrals, which often indicate that you're hitting an ISP's limits. Adjust your settings incrementally, observing the impact on your deliverability and throughput. The goal is to maximize your sending speed without incurring penalties.
Example Postfix configuration for domain limitsplaintext
Email authentication protocols, specifically SPF, DKIM, and DMARC, are fundamental to building and maintaining a strong sender reputation. These protocols verify that your emails are legitimate and sent from authorized sources, significantly increasing ISP trust.
ISPs use authentication results as a key signal for determining how to handle your mail, including how many connections they'll allow. Properly configured authentication makes it easier for ISPs to trust your sending domain, potentially allowing for higher connection limits and reducing the likelihood of throttling or being placed on a blacklist.
Beyond technical configurations, maintaining a clean and engaged email list is paramount. Sending to invalid or unengaged addresses generates bounces and spam complaints, which directly hurt your sender reputation. Regularly cleaning your list and focusing on sending relevant content to opted-in subscribers will lead to better engagement metrics, which ISPs value highly.
Key authentication measures
SPF: Authorizes specific IP addresses to send mail on behalf of your domain.
DKIM: Adds a digital signature to your emails, verifying content integrity during transit.
DMARC: Specifies how receivers should handle emails that fail SPF or DKIM authentication.
Final thoughts on optimizing email sending
Mastering email domain connection limits is a continuous process of observation and adjustment. It is not about finding a single magic number, but rather understanding that these limits are fluid and directly tied to your sender reputation and how ISPs perceive your sending behavior.
By focusing on strong authentication, gradual IP warming, and maintaining a high-quality email program, you can navigate ISP restrictions effectively. Consistent monitoring and an agile approach to adjusting your sending parameters will ensure your emails reach the inbox reliably.
Views from the trenches
Best practices
Always implement SPF, DKIM, and DMARC to authenticate your emails and build sender trust with ISPs.
Start with conservative connection and message limits, then gradually increase based on deliverability metrics.
Regularly monitor bounce logs and ISP feedback loops to identify and resolve any throttling issues promptly.
Maintain clean email lists by removing inactive and bounced addresses to prevent reputation damage.
Common pitfalls
Setting unlimited connections can trigger ISP spam filters and lead to immediate throttling or blocking.
Ignoring slow delivery rates, which are often early indicators of hitting connection limits or reputation issues.
Failing to warm up new sending IPs, resulting in abrupt volume increases that ISPs deem suspicious.
Not aligning all sending domains and subdomains with proper authentication protocols.
Expert tips
Use dedicated IP addresses to have more control over your sender reputation and avoid shared pool issues.
Leverage Postmaster Tools from major ISPs like Google and Microsoft for valuable insights into your sending performance.
Segment your audience and personalize content to improve engagement, which positively impacts sender reputation.
Consider using an email service provider (ESP) or robust MTA software for advanced limit management and analytics.
Marketer view
Marketer from Email Geeks says ISPs will often throttle if you attempt too many simultaneous connections within a short period, and they typically don't provide clear guidelines on acceptable limits.
2018-05-16 - Email Geeks
Marketer view
Marketer from Email Geeks says finding explicit material on connection limits for major domains like Outlook and Gmail is difficult because they adjust dynamically based on your reputation.
Gmail and 
Outlook) might interpret an excessive number of simultaneous connections or messages per connection as a sign of spamming. This can lead to throttling, where your emails are intentionally delayed, or even outright blocking, which harms your sender reputation.
Google and 
Microsoft. 
