What causes 'Server is not currently available, possibly due to too many connections' and how to fix it?
Matthew Whittaker
Co-founder & CTO, Suped
Published 12 May 2025
Updated 19 Aug 2025
8 min read
Encountering the error message 'Server is not currently available, possibly due to too many connections' can be frustrating for anyone sending emails. It's a clear signal that the receiving mail server is overwhelmed or intentionally limiting incoming connections. This issue often leads to delayed or failed email deliveries, directly impacting your communication flow and potentially your sender reputation. Understanding the root causes of this problem is the first step toward effective troubleshooting and ensuring your emails reach their intended recipients.
This error isn't exclusive to email, it's a common server-side issue that indicates a resource exhaustion problem, meaning the server cannot accept new connections because its capacity has been reached. In the context of email, this often points to rate limiting, aggressive spam filtering, or genuine server overload on the recipient's end.
While the message itself might seem straightforward, the underlying reasons can be complex, ranging from your own sending practices to the recipient's server configurations. We'll delve into the specific triggers for this error in email environments and outline the steps you can take to diagnose and resolve it, ensuring smoother email operations.
Understanding the 'Too many connections' error in email
When your email server receives a 'Server is not currently available, possibly due to too many connections' message, it's typically an SMTP 421 transient error. This means the server expects you to retry the delivery later. However, if these retries consistently fail or if the error persists, it becomes a significant delivery issue. It signifies that the recipient's mail server has reached its maximum allowable concurrent connections or is undergoing maintenance. This situation can also lead to a 421 service not available error, indicating similar underlying problems.
This error is distinct from a hard bounce, which indicates a permanent failure. A 421 error is a soft bounce, implying a temporary issue. Mail servers are usually configured to queue messages and attempt re-delivery. However, if the server remains unavailable for an extended period, the message might eventually time out and bounce back as undeliverable. This is particularly relevant when dealing with specific mailbox providers that enforce strict connection limits, such as Hotmail.
The message implies a transient state, but persistent occurrences can signal deeper problems like poor sender reputation or being placed on a blocklist. Sometimes, it's a direct consequence of a mail server temporarily rejecting messages due to perceived suspicious activity or volume. It's crucial to differentiate between a temporary glitch and a recurring issue that needs immediate attention.
Common causes of connection errors for email servers
Several factors can contribute to a 'too many connections' error. High email volume from a single IP address is a primary culprit. If you're sending a large number of emails in a short period, the recipient server might interpret this as spamming activity and throttle your connections to protect its resources. This is a form of rate limiting designed to prevent server overload and abuse.
Misconfigured sending applications or mail servers can also lead to this error. If your sending system isn't properly closing connections after each email transmission or is attempting to open too many simultaneous connections, it can quickly exhaust the recipient server's available slots. This is a common issue with Postfix configurations, for instance.
Finally, the recipient's server might genuinely be experiencing an outage, high legitimate traffic, or even a distributed denial-of-service (DDoS) attack. In such cases, the server becomes inaccessible or operates at a reduced capacity, leading to connection failures for all incoming requests. The 503 Service Unavailable HTTP status code is a general server-side error that reflects similar conditions.
Beware of generic PTR records
A common but often overlooked cause, as seen in the community, is an invalid or generic PTR (reverse DNS) record. Some mailbox providers, like Proofpoint, will defer or block connections from IPs with generic PTRs, as they prefer fully qualified domain names (FQDNs) that resolve back to the IP. Ensure your PTR record designates clear ownership and authorized use, typically starting with common mail server prefixes such as 'mail', 'mts', 'mx', 'out', or 'smtp'.
Diagnosing the 'Too many connections' error
To effectively address this error, you need to diagnose its specific cause. Start by checking your mail server logs. These logs provide detailed information about why a connection failed, often including the exact error message received from the recipient server. Look for patterns in the errors, such as specific recipient domains that consistently return this message, like Charter.net.
Next, evaluate your sending volume and rate. Are you sending a sudden burst of emails? Are your parallel connection limits too high for certain recipient domains? Many mailbox providers have undocumented or dynamic rate limits. Exceeding these limits, even with genuine traffic, can lead to temporary blocks. Consider monitoring your email deliverability closely to detect such patterns.
Also, verify the status of your IP address on various email blacklists (or blocklists). While the error message doesn't directly indicate a blacklist, being listed can significantly impact a server's willingness to accept your connections. A generic PTR record, for example, could contribute to a lower reputation score, making servers more likely to trigger connection-related errors or even block your dedicated IPs.
Immediate and short-term solutions
For immediate relief, the most common solution is to reduce your parallel connections and sending rate to the problematic server. Many mail transfer agents (MTAs) allow you to configure specific sending limits per domain. Gradually increasing these limits after successful delivery can help you find the optimal rate without triggering the error again.
Example Postfix configuration for reducing connectionsini
Ensure your mail server's retry queue is configured to handle deferred messages appropriately. A robust retry mechanism will reattempt delivery after a specified delay, giving the recipient server time to recover. However, be mindful of the retry intervals, trying too frequently can exacerbate the issue.
If you suspect a PTR record issue, as outlined by Proofpoint support, update it to a fully qualified domain name that resolves back to your IP. While this might not solve a 'too many connections' error directly, it's a fundamental part of maintaining good sender reputation and can prevent other delivery issues. After updating, allow for DNS propagation time and then re-evaluate.
Long-term strategies for prevention
Reactive measures
Adjusting sending limits: Manually lowering parallel connections and sending rates per domain or IP.
Temporary suspension: Pausing campaigns to affected domains until the issue resolves.
Proactive prevention
Gradual warming: Gradually increasing sending volume to new IPs or domains to build reputation.
Reputation management: Maintaining a healthy sender reputation through clean lists and authentication like DMARC. This is key to preventing emails from going to spam.
Infrastructure scaling: Ensuring your mail infrastructure can handle anticipated sending volumes without overwhelming recipient servers.
To prevent 'too many connections' errors from recurring, implement a robust email delivery strategy. This includes segmenting your email lists, warming up new IPs or domains gradually, and distributing your sending volume over time rather than in large bursts. Consistent, measured sending helps maintain a good sender reputation with mailbox providers. Consider using different technical solutions from top performing senders.
Regularly review your email authentication records, including SPF and DKIM, and especially your PTR record. Ensuring these are correctly configured and consistent helps recipient servers trust your outgoing mail, reducing the likelihood of them rate-limiting or deferring your connections. Remember, a generic PTR record can contribute to perceived untrustworthiness.
Code
Meaning
Common causes related to connections
421
Service not available, closing transmission channel
Recipient server overloaded, undergoing maintenance, or imposing rate limits.
451
Requested action aborted: local error in processing
Requested action not taken: insufficient system storage
Recipient server lacks disk space, often a precursor to or symptom of being overwhelmed.
Views from the trenches
Best practices
Implement staggered sending schedules to distribute email volume over longer periods.
Maintain clean email lists to minimize bounces and reduce unwanted traffic.
Regularly review your email authentication records like SPF, DKIM, and PTR for correctness.
Monitor your delivery logs closely for patterns of temporary errors from specific mailbox providers.
Gradually warm up new sending IPs or domains before sending large volumes.
Common pitfalls
Sending a sudden large volume of emails to a new or unfamiliar recipient domain.
Not configuring a proper reverse DNS (PTR) record for your sending IPs.
Ignoring recurring 4xx transient errors, assuming they will always resolve on their own.
Setting excessively high parallel connection limits on your mail server.
Failing to close SMTP connections promptly after sending, exhausting server resources.
Expert tips
Engage directly with postmasters of frequently problematic domains if issues persist.
Utilize DMARC reporting to gain insights into delivery issues and authentication failures.
Consider a dedicated IP address for high-volume sending to better control your reputation.
Segment your audience and tailor sending rates based on recipient domain behavior.
Automate dynamic rate limiting based on real-time server responses.
Marketer view
Marketer from Email Geeks says they were facing deferred messages with a 421 4.7.0 error code, even after confirming their IPs were not listed on Proofpoint.
2022-11-18 - Email Geeks
Marketer view
Marketer from Email Geeks says that even after fixing generic PTR records to fully qualified domain names, they continued to experience 'too many connections' issues, especially with Charter.net, despite reducing parallel connections to 2.
2022-11-20 - Email Geeks
Maintaining healthy email delivery
The 'Server is not currently available, possibly due to too many connections' error is a common but manageable challenge in email delivery. It underscores the importance of understanding not only your own sending infrastructure but also the nuances of recipient server policies and limitations.
By proactively managing your sending reputation, optimizing your mail server configurations, and employing smart sending practices, you can significantly reduce the occurrence of this error. Staying vigilant with your logs and adapting your strategy to individual recipient domains are crucial for maintaining high deliverability rates. This proactive approach is a cornerstone of improving email deliverability.
Proofpoint, will defer or block connections from IPs with generic PTRs, as they prefer fully qualified domain names (FQDNs) that resolve back to the IP. Ensure your PTR record designates clear ownership and authorized use, typically starting with common mail server prefixes such as 'mail', 'mts', 'mx', 'out', or 'smtp'.
Charter.net.
