Summary
Temporary server errors on Office 365 for specific subnets can be caused by a multitude of factors ranging from reputation issues, authentication problems and transient issues with SMTP connections. Reputation issues arise from other bad actors using the same subnet, or because that subnet is listed in the Talos database, Microsofts internal database. Authentication issues can result in Office365 temporarily rejecting connections. Implementing proper retry logic with exponential backoff is crucial to handle these transient issues without overwhelming the receiving server.
Key findings
- Reputation: IPs used on a subnet may have a history of low engagement, spam or other issues.
- Throttling: Office365 may be enforcing subnet specific rate limiting.
- Authentication: Incorrect DKIM/SPF records will cause rejections
- Limits: Office365 enforces rate limits, which result in temporary errors.
- Blacklisting: An IP on a subnet could be listed on an external blacklist.
- Network Issues: Temporary server errors could be a symptom of network, or SMTP issues.
Key considerations
- Reputation: Investigate IP reputation and check against common blacklists.
- Authentication: Validate SPF/DKIM records for all sending domains.
- Configure: Examine MTA for TLS, EHLO and rDNS issues.
- Greylisting: Be aware that greylisting is a potential cause, and legitimate servers will attempt to send again.
- Limits: Implement retry logic in your sending infrastructure to manage limits.
- Warming: New or low sending IPs will require warming to build trust.
What email marketers say
11 marketer opinions
Temporary server errors on Office 365 for specific subnets often indicate problems with IP reputation, subnet-specific throttling, or authentication issues. Potential causes include: historical problems on Talos data feed (used by Microsoft), subnet blocks, incorrect SPF/DKIM records, shared IP issues, missing or mismatched rDNS records, rate limits, low IP sending volume, or listing on blocklists. Greylisting may also play a factor. Use online blacklist checkers to check the IP's reputation.
Key opinions
- Reputation Issues: Subnets may have a poor IP reputation due to spam or low engagement, causing throttling or blocking by Office 365.
- Subnet Throttling: Office 365 may implement subnet-specific throttling due to perceived abuse from the subnet.
- Authentication Problems: Incorrect SPF/DKIM records or mismatched rDNS records can cause temporary rejections.
- Rate Limits: Exceeding Office 365's rate limits for incoming emails from specific sources (subnets) triggers temporary errors.
- Shared IPs: If using shared IPs, other senders can negatively impact deliverability.
- Blacklisting: The sender IP might be listed on public email blocklists.
Key considerations
- Check Reputation: Use tools to check the IP/subnet reputation and blocklist status.
- Verify Authentication: Ensure SPF/DKIM records are correctly configured for all sending IPs within the subnet, and rDNS records match the forward DNS.
- Monitor Sending Volume: If a new IP, gradually warm up the IP to build trust.
- Investigate Subnet Activity: If using shared IPs, investigate the sending practices of others on the subnet.
- Review limits: Confirm if rate limits are configured correctly
- Implement Retries: Make sure retry logic is working to recover from temporary errors.
Marketer view
Email marketer from Mailjet shares that new IPs or IPs with low sending volume can encounter temporary server errors due to trust issues. Warming up IPs gradually by increasing sending volume over time can improve deliverability.
6 Feb 2024 - Mailjet
Marketer view
Email marketer from StackOverflow explains that Office 365 imposes rate limits on incoming emails from specific sources (subnets). If you exceed these limits, you might encounter temporary server errors until the rate limit resets.
7 Jun 2025 - StackOverflow
What the experts say
4 expert opinions
Temporary server errors on Office 365 for specific subnets can stem from various issues. It's advised to use a working subnet as a temporary workaround while investigating the root cause. Potential problems include greylisting, where legitimate emails are temporarily rejected, and subnet reputation issues, especially in cloud environments, where the actions of other users on the same subnet can negatively impact your sending reputation. Expert's also suggest closely examining your MTA configuration, including TLS negotiation, EHLO values, and rDNS records.
Key opinions
- Temporary Issue: The error might be genuinely temporary, requiring a retry mechanism.
- Greylisting: Office 365 might be using greylisting, causing initial delivery attempts to fail.
- Subnet Reputation: The subnet's reputation may be poor due to the activities of other senders on the same subnet.
- MTA Configuration: There might be issues with the Mail Transfer Agent (MTA) configuration.
Key considerations
- Workaround: Use a different, functional subnet as a temporary solution.
- Retry Mechanism: Implement a retry mechanism to handle temporary rejections due to greylisting or other temporary issues.
- Monitor Reputation: Monitor the subnet's reputation and identify any potentially problematic senders on the same subnet.
- Examine MTA: Carefully review the MTA configuration, checking TLS negotiation, EHLO values, and rDNS records.
Expert view
Expert from Spam Resource explains that greylisting, a spam-fighting technique, could be the cause. Greylisting works by temporarily rejecting emails from unknown senders. A legitimate server will try again after a delay and be accepted, while a spammer may not.
11 Jun 2023 - Spam Resource
Expert view
Expert from Word to the Wise shares that subnet reputation can be problematic, especially with cloud providers. If other users on your subnet are sending spam, it can negatively impact the reputation of the entire subnet. This means temporary errors from services like 0365.
15 Jan 2022 - Word to the Wise
What the documentation says
5 technical articles
Temporary server errors in Office 365, indicated by 4xx or 5xx SMTP error codes, are often caused by transient network issues, server overload, maintenance activities, or DNS resolution problems. These errors indicate the sender should retry delivery. Implementing proper retry logic with exponential backoff is crucial to handle these transient issues without overwhelming the receiving server.
Key findings
- Transient Network Issues: Temporary errors can arise from transient network problems.
- Server Overload/Maintenance: Office 365 servers may be overloaded or undergoing maintenance, leading to temporary errors.
- DNS Resolution Problems: Inability to reliably resolve MX records for the recipient domain can cause temporary errors.
- SMTP Connection Issues: Transient issues with SMTP connections, such as network congestion or brief outages, can trigger these errors.
- Retry Recommended: SMTP servers experiencing temporary failures will provide a 4xx error code indicating the client should retry the request.
Key considerations
- Implement Retry Logic: Ensure your email sending infrastructure implements robust retry logic to handle temporary server errors.
- Exponential Backoff: Use an exponential backoff strategy when retrying to avoid overwhelming the receiving server.
- Monitor Network Stability: Monitor network connections to reduce transient network errors
- DNS Resolution Stability: Ensure that DNS servers are properly configured and responding reliably to reduce DNS resolution issues.
Technical article
Documentation from Microsoft Learn explains that temporary errors in Exchange Online, often indicated by 4xx or 5xx SMTP error codes, can arise from transient network issues, server overload, or maintenance activities. These errors suggest the sender should retry delivery after a certain period.
26 Apr 2024 - Microsoft Learn
Technical article
Documentation from SendGrid explains that implementing proper retry logic in your email sending infrastructure is essential to handle temporary server errors. This involves automatically retrying failed deliveries after a delay, with an exponential backoff strategy to avoid overwhelming the receiving server.
18 Jan 2024 - SendGrid Documentation
Are there email sending issues with AWS?
Does UCE Protect Level 3 at an ESP affect delivery to major ISPs like Hotmail or Office 365?
Does UCEPROTECTL3 listing impact email deliverability, especially with Microsoft Office 365?
How can I improve email deliverability with Microsoft and avoid spam filters?
How can I resolve Microsoft Outlook S3140 errors blocking my transactional emails?
How do I resolve a blocked sending IP with Office365 and what steps should I take to ensure transparency?
