What is the SMTP code 25?

The SMTP code 25 indicates an issue with connecting to the recipient’s mail server. Specifically, it means the connection was rejected by the receiving server due to some sort of error.

Some potential causes of SMTP code 25 errors include:

  1. The recipient’s email address is not valid or does not exist on the destination mail server.
  2. The receiving mail server is rejecting connections for security reasons, like too many connections from the same IP address.

There are temporary issues on the receiving mail server preventing connections, like maintenance or resource limits being reached.
What is the SMTP code 25?

What causes SMTP code 25 errors?

There are a few potential root causes of SMTP 25 errors:

  • Invalid recipient email address – If the email address you’re trying to send mail to doesn’t actually exist on the destination mail server, that mail server will reject connections with a 25 error. Double check that the address is typed correctly.
  • Security policies on destination server – Mail servers have security mechanisms in place to prevent issues like spam and denial of service attacks. If connections from your mail server IP exceed thresholds for things like number of messages or connection requests per minute, the receiving server may temporarily block connections with a 25 error.
  • Resource issues on destination server – Sometimes 25 errors indicate a resource constraint on the destination mail server. If the server is out of disk space, running low on memory, or there are too many simultaneous connections, it might fail to accept your connection attempt.
  • Transient network issues – Network interruptions between the two mail servers involved can also cause 25 errors by preventing connections. The same connection attempt made again later may succeed if the network issue was temporary.

How to troubleshoot and prevent SMTP code 25

There are a few things you can try to troubleshoot, resolve, or prevent SMTP 25 errors:

Verify recipient addresses – Double check that all recipient email addresses are typed correctly without typos. Also confirm that the specific mailboxes exist on the destination domain by contacting their email administrator if needed.

Review receiving server policies – Check with the administrator of the receiving mail server to understand their security policies, limits, and blacklisting procedures. Then make sure your mail server activities stay within defined thresholds.

Try again later – If the issue seems tied to transient network or server problems, retry sending the messages after some time has passed to see if connections now succeed without a 25 error.

Use DNS lookups – Enable reverse DNS hostname lookups on your mail server and configure it to reject invalid or suspicious hostnames that can’t be verified. This prevents backscatter issues.

Invest in hardware – If your own mail server seems to struggle with resource constraints causing 25 errors under high load, consider expanded hardware capacity to handle increased volume.

Implement throttling rules – Adding throttling policies that limit the number of outbound connection attempts per minute/hour can avoid triggering recipient server security rules.

By identifying the root cause and making appropriate configuration changes or upgrades, SMTP 25 errors can typically be resolved.

Key takeaways about SMTP code 25

  • An SMTP 25 error means the receiving mail server rejected the connection attempt.
  • This is often due to an invalid recipient address, but it can also be from security policies, transient network issues, or resource constraints on destination server.
  • Troubleshooting involves verifying addresses are valid, contacting the destination mail administrator to understand their policies, retrying the message transmission, and potentially upgrading sending mail server hardware capacity.
  • Carefully managing mail server connection throttling and implementing reverse DNS lookups helps avoid triggering recipient server 25 errors.


In summary, SMTP code 25 errors signify an issue establishing a connection with the intended recipient’s mail server. There are ways to troubleshoot and prevent 25 errors on both the sending and receiving mail server side, centered on understanding the reason for the rejection and addressing it through appropriate configuration adjustments or capacity expansion.

With robust mail server infrastructure and policies in place that align with destination server requirements, 25 errors can typically be avoided and email delivery success maximized.

Frequently Asked Questions About SMTP Code 25

  1. What does SMTP code 25 mean?
    SMTP code 25 means the connection attempt to the receiving mail server was rejected. It indicates there is an issue with the validity of the recipient address, security policies on the destination server, network connectivity, or resource constraints preventing connections.
  2. Why do I get SMTP 25 errors?
    Common reasons for 25 errors include having an invalid recipient address with a typo, trying to send too many messages too quickly from a single IP triggering security blocks on the receiving server, network outages between mail servers, or inadequate server resources on recipient side.
  3. How do I fix SMTP 25 issues?
    Fixes involve verifying recipient addresses are typed correctly and exist on destination domain, reviewing and aligning with receiving server connection limit policies, retrying message send later if transient network or resource usage issue, upgrading sending mail server capacity if needed, and implementing throttling rules.
  4. What’s the difference between SMTP 25 and SMTP 550?
    Code 25 signifies rejection due to connection issues with the destination server. Code 550 indicates the recipient address itself is invalid according to the structure and format required by the receiving mail server.
  5. Can SMTP code 25 be ignored?
    No – A 25 error means delivery failure to that recipient. It should be addressed by troubleshooting why the receiving system rejected the connection, such as an invalid recipient or their server security policies blocking you.
  6. What does SMTP 25 unable to relay mean?
    The “unable to relay” addition to a 25 error means the receiving server rejected the connection specifically because the sender failed authentication and is not considered a trusted party allowed to relay mail through their infrastructure.
  7. Is SMTP error 25 permanent?
    Not always – Sometimes a code 25 signifies a temporary issue like network instability or peaked resource usage levels on the recipient’s system. But it can also indicate a permanent invalid address or blocked sending IP, requiring configuration changes before further delivery attempts.
  8. What is SMTP code 25 in Exchange?
    In Microsoft Exchange, error 25 most typically maps to connections being rejected due to SMTP protocol-level transport limits, content restrictions, or recipient address validity checks configured within Exchange itself.
  9. Can SMTP code 25 cause bounce backs?
    Yes – The failed connection attempt that generates a 25 error essentially guarantees the original message will ultimately bounce back to the sender since it was not accepted by the receiving domain.
  10. Why am I getting hundreds of SMTP 25 errors?
    Getting a high volume of SMTP 25 errors likely means there is an issue with the sender’s system configuration or software that is repeatedly trying and failing to establish SMTP connections with many destination servers. Identifying this pattern is key to resolving it at the root cause.
  11. What should my SMPT retry policy be after 25?
    A reasonable retry policy after getting SMTP error 25 would be attempting two reconnect attempts almost immediately in case it was a transient error, then implement an exponential backoff of retrying after 5 minutes, 10 minutes, 30 minutes and finally 1 hour before giving up.
  12. Is there a workaround for SMTP 25 connection refused?
    Workarounds depend on the exact cause – If security policy thresholds are being exceeded, implement connection throttling rules to avoid that happening. For invalid addresses causing 25 errors, double check and correct those. For destination server resource issues consider scheduling large mailings during off-peak hours.
  13. Can I whitelist IPs to avoid SMTP 25?
    Yes, requesting your mail server’s IPs be explicitly whitelisted or added as a trusted relay on external domains you frequently communicate with is a good way to avoid 25 errors related to receiving server security policies.
  14. Why do I get SMTP 25 when sending to Gmail?
    Gmail has notoriously strict delivery throttling policies and spam filters enforced by Google to protect their users. As a result connections from unfamiliar servers often face 25 errors until requesting whitelisting for dedicated IPs to prove non-malicious intent over time.
  15. Does SMTP 25 mean I’m blacklisted?
    Possibly – If the connections are failing consistently with a 25 specifically when attempting to communicate with just certain domains, it can signify your IPs have ended up on a real-time blacklist. You’ll need to determine why, correct it, then request removal from affected blacklists.
  16. Can my IP get banned from too many 25 errors?
    Absolutely – Sending servers generating frequent 25 errors run the substantial risk of flagged as suspicious by receiving servers and having their IPs outright banned from connecting. This amplifies the importance of properly handling and troubleshooting 25 issues.
  17. Should I increase SMTP timeout after 25 errors?
    It’s reasonable to slightly boost SMTP timeout thresholds after running into multiple 25 errors, under the assumption that increased timeouts may allow some additional time for transient network flakiness or destination server recover from any brief resource constraints. But overly long SMTP timeouts can cause other issues.
  18. What are SMTP 25 best practices?
    Best practices around avoiding 25 errors include properly validating recipient addresses, carefully pacing connection volumes and message frequency to align with recipient server limits, implementing reverse DNS and SPF records, having robust mail server infrastructure, establishing trusted partnerships with destination domains, and promptly troubleshooting any 25 errors that do occur.

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