What are common connection and message limits imposed by different email providers?
Michael Ko
Co-founder & CEO, Suped
Published 4 May 2025
Updated 19 Aug 2025
10 min read
Navigating the world of email deliverability often feels like walking through a minefield of hidden rules and unspoken boundaries. One of the most critical aspects to understand, especially for anyone sending emails in volume, is the various limits imposed by different email providers. These aren't just arbitrary numbers, but carefully set thresholds designed to protect their users from spam and maintain the health of their networks.
I've seen many senders stumble because they underestimated these restrictions, leading to email throttling, messages bouncing, or even getting their sending IP address (or domain) blocklisted. It's a common misconception that if your email is legitimate, it will always go through. Unfortunately, that's not always the case if you're hitting rate limits or connection caps.
Understanding these limits is fundamental to ensuring your emails reach the inbox. It's not just about the raw number of emails you send, but also how you send them, how many recipients are on a single message, and how many concurrent connections you open. Each provider has its own nuances, and what works for one might not work for another.
Understanding different types of limits
Email service providers (ESPs) and mailbox providers implement various types of limits to manage server load, prevent abuse, and ensure a good user experience. These limits fall into several categories, each impacting how your emails are processed.
The most widely known limits are related to the volume of emails sent per day or hour. For instance, a free Gmail account typically allows up to 500 emails per day when sent via the browser. Business accounts (Google Workspace) have significantly higher limits, often around 2,000 to 10,000 emails daily, depending on the plan. These limits can apply per mailbox or per domain, a crucial distinction for bulk senders.
Microsoft, with its Exchange Online service, also enforces strict policies. For Microsoft 365 users, the daily limit can be up to 10,000 recipients. However, it's important to note that these limits are often coupled with recipient limits per message and rate limits per minute. For example, you might be allowed to send to 500 recipients per message and 30 messages per minute. Hitting these more granular limits can lead to throttling or delays.
Yahoo! Mail, a significant player, tends to have lower daily sending limits for free accounts, often around 100 emails. Their SMTP connection and concurrent link limits are also critical. While older limits might have been 5 messages per session before a connection reset, they later increased to 20, which was a notable change at the time. Understanding why Yahoo might be throttling email messages and its common solutions is key.
Recipient, connection, and size limitations
Beyond the sheer volume of emails, providers impose limits on the number of recipients per message. This is a common way to curb spam and prevent users from sending large mailing lists from a standard email client. For example, Microsoft 365 allows up to 500 recipients per single email, while free Gmail accounts might be restricted to 100 recipients per email when sent via SMTP.
Connection limits are also a big one. These determine how many simultaneous connections your sending server can establish with the recipient's mail server. Many providers limit the number of concurrent connections from a single IP address or IP block. If you exceed this, you'll often see deferrals or temporary rejections. This is particularly relevant for managing concurrent outbound connections to major ISPs. Some niche providers, especially in Europe and China, have very low concurrent connection limits, sometimes as low as one connection per outbound IP. This can significantly impact throughput, even for perfectly legitimate mail.
Message size and attachment limits are also common, though often overlooked. Most email services set a maximum total email size, which includes both the message body and attachments. While there's no universal standard, a common maximum is around 25 MB, with some being stricter. Exceeding this will result in a bounce, and you'll receive a delivery failure notification. This is why it's important to understand the attachment and message size limits for different mailbox providers.
Beyond the explicitly stated limits, some providers also implement rate limits per session, meaning they will only accept a certain number of emails over a single connection before closing it or initiating a temporary block. This is often dynamic and based on your sender reputation. A good sender might have a higher limit per session, while a questionable one might have it drastically reduced, perhaps to just a few messages.
Consequences of exceeding limits
Understanding the limits
Each major provider, such as Google, Microsoft, and Yahoo, publishes guidelines for bulk senders and general usage. It's crucial to consult these official sources, as limits can change over time. For example, Gmail's delivery rate limits are heavily influenced by sender reputation.
Dynamic nature
Limits are not always static. They often adapt based on your sending reputation, the type of content you send, and recipient engagement. A sudden increase in volume from an unestablished IP, for example, will likely trigger more aggressive rate limiting than consistent, measured sending from a well-reputed IP. This makes IP warming crucial for new senders or IPs.
Hitting these limits usually results in temporary deferrals (like a 4xx SMTP error code). These are signals that the receiving server is telling you to slow down or adjust your sending behavior. Persistently ignoring these signals or aggressively trying to bypass them can lead to more severe consequences, such as being placed on a blacklist or blocklist (or blocklist), which can severely damage your deliverability.
For instance, if you're sending to Optimum/Optonline recipients and encountering connection limit issues, it signifies a need to adjust your sending strategy. Similarly, if Hotmail emails are failing due to connection limits, it's a clear indicator that your current volume or rate is too high for their current policies regarding your IP/domain.
One common problem I see is that many senders only focus on the total daily sending limits and neglect the more granular hourly, minute, or even per-connection limits. Many providers, especially smaller or regional ones, will enforce stricter limits on these smaller timeframes or per-connection. For example, some might accept only 20 messages per session before closing the connection. This is a crucial detail, especially for those who manage their own mail transfer agents (MTAs).
Strategies to optimize and stay within limits
Optimizing your sending to respect these limits is about more than just staying off a blacklist (or blocklist). It's about building and maintaining a good sender reputation, which ultimately leads to better inbox placement. Here's how to approach it:
Start small and warm up: If you're using a new IP or domain, gradually increase your sending volume over time. This process, known as IP warming, helps mailbox providers recognize you as a legitimate sender.
Monitor delivery errors: Pay close attention to bounce messages and SMTP error codes. A 421 or 451 error often indicates a temporary rejection due to rate limiting or connection limits. Use this feedback to adjust your sending speed.
Distribute your sends: Instead of sending all your emails at once, spread them out over hours or days. This smooths out your sending pattern and makes you look less like a spammer.
Use an email service provider (ESP): Professional ESPs are designed to manage sending infrastructure, IP reputation, and adhere to various provider limits, taking much of the burden off your shoulders. They handle the complex task of handling email sending rates and connection limits.
Some common SMTP reply codes provide clues about why your emails are being limited. Recognizing these can help you react quickly and adjust your sending strategy.
SMTP reply code
Meaning
Action to take
421
Service not available, closing transmission channel. Often a temporary rate limit.
Reduce sending rate immediately. Wait and retry.
451
Requested action aborted: local error in processing. Can indicate overloaded servers or policy violations.
Slow down your sending. Investigate recipient domain health.
550
Requested action not taken: mailbox unavailable, or policy violation.
This is often a hard bounce. Remove invalid addresses from your list.
554
Transaction failed, often due to spam detection or blocklisting (blacklist) issues.
Understanding and responding to these codes is a critical aspect of managing your email deliverability effectively. Ignoring them can lead to prolonged delivery issues and damage to your sender reputation.
The dynamic nature of limits and continuous monitoring
In the ongoing battle against spam, email providers are constantly adjusting their algorithms and limits. What works today might not work tomorrow. It's an evolving landscape, and staying informed is crucial for any sender.
This constant change can be frustrating, especially for legitimate senders who are trying to adhere to best practices. Mailbox providers don't always publish real-time updates to their granular limits, as doing so could potentially be exploited by bad actors. Instead, they often rely on dynamic adjustments based on real-time traffic and sender reputation metrics.
This is why monitoring your deliverability and bounce logs is paramount. The feedback loop from the receiving servers is the most accurate way to understand the current limits being applied to your sending. If you notice a sudden increase in 4xx errors, it's a strong indication that you're hitting a temporary limit and need to ease off your sending rate.
Views from the trenches
Best practices
Maintain a healthy sender reputation by sending relevant content to engaged recipients.
Always clean your email lists regularly to remove inactive or invalid addresses, reducing bounces.
Implement strong email authentication (SPF, DKIM, DMARC) to build trust with mailbox providers.
Segment your audience and tailor content to improve engagement and reduce complaint rates.
Common pitfalls
Ignoring SMTP 4xx temporary error codes and continuing to send at high volumes.
Sending to unengaged lists, leading to high bounce rates and spam complaints.
Not warming up new IPs or domains, resulting in immediate throttling or blocks.
Overlooking recipient-per-message limits by trying to send to too many people at once.
Expert tips
Monitor your bounce logs carefully; 4XX errors, especially those with specific text, provide real-time feedback on current rate limits.
For very large sends, consider geographically distributing your sending infrastructure to leverage different IP ranges and connection limits.
Engage with the deliverability community; anecdotal evidence and shared experiences can offer insights into unpublicized limits.
Continuously test your email deliverability across major providers to identify potential issues before they escalate.
Marketer view
Marketer from Email Geeks says that large platforms are constantly changing their limits, which makes it hard for legitimate senders to keep up.
March 19, 2021 - Email Geeks
Expert view
Expert from Email Geeks says that significant effort is dedicated to managing large volumes of legitimate outbound email, making dynamic limit information useful for good senders.
March 19, 2021 - Email Geeks
Final thoughts on email limits
Navigating the common connection and message limits imposed by different email providers is an essential aspect of successful email deliverability. These limits, whether daily sending caps, recipients per message, concurrent connections, or message size, are designed to protect the integrity of the email ecosystem.
By understanding the typical limits from major players like GoogleMicrosoftYahoo, and others, and by continuously monitoring your sending performance, you can proactively adjust your strategy to avoid throttling, bounces, and the dreaded blacklist (or blocklist).
Remember, good sending behavior isn't just about avoiding penalties, it's about building and maintaining a positive sender reputation that ensures your legitimate emails consistently reach their intended inboxes. Adhering to these limits is a cornerstone of that process.
Gmail accounts might be restricted to 100 recipients per email when sent via SMTP.
Google, 
Microsoft, and 
Yahoo, publishes guidelines for bulk senders and general usage. It's crucial to consult these official sources, as limits can change over time. For example, Gmail's delivery rate limits are heavily influenced by sender reputation.
