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What does 'Unable to Read SMTP Banner, connection lost' mean?

6 Marketer opinions6 Expert opinions4 Technical articles3 Resources

Summary

The 'Unable to Read SMTP Banner, connection lost' error signifies a failure in the initial connection phase between mail servers. Experts and documentation indicate various potential causes, primarily centered around network-level issues. These encompass general network failures, firewall restrictions blocking SMTP traffic, and the remote server being unavailable (due to overload, maintenance, or other problems). The error arises before the HELO command is exchanged, during the TCP handshake when the sending server expects to receive the '220 Service Ready' greeting. Other contributing factors include DNS misconfiguration, incorrect SMTP port usage, sender IP blacklisting, and problems with the SMTP server's configuration or banner settings. Due to the transient nature of some of these causes, retrying the connection is often advised, along with a thorough check of network connectivity, firewall rules, and server status.

Key findings

  • Initial Connection Failure: The error indicates a failure to establish a TCP connection and receive the initial SMTP banner from the remote server.
  • Network-Level Problems: Network outages, congestion, and firewall restrictions are prominent causes.
  • Server Unavailability: The remote server might be offline, overloaded, or experiencing issues that prevent it from responding to connection attempts.
  • Pre-HELO Stage: The error occurs before the SMTP HELO command, during the TCP handshake and banner exchange.
  • Configuration Issues: SMTP server configuration problems, incorrect port usage, DNS misconfiguration, and sender IP blacklisting can also contribute to the error.

Key considerations

  • Check Network Connectivity: Verify network connectivity between the sending and receiving mail servers to rule out basic network issues.
  • Review Firewall Rules: Ensure that firewalls are not blocking SMTP traffic on port 25 (or other configured SMTP port) and that the sending server's IP is not blocked.
  • Monitor Remote Server Status: Check the status of the remote server to confirm it is online and accepting connections.
  • Diagnose DNS Issues: Verify that DNS is correctly resolving the remote server's hostname to an IP address.
  • Inspect Blacklist Status: Check the sending server's IP address against known blacklists to ensure it is not being blocked by recipient mail servers.
  • Retry Connection: If the error appears to be transient, retry the connection after a short delay to see if the issue resolves itself.
  • Check SMTP Configuration: Check SMTP configuration to make sure all of the configuration is correct

What email marketers say
6Marketer opinions

The error 'Unable to Read SMTP Banner, connection lost' generally indicates a failure to establish a connection with the recipient mail server. This can stem from various network-related issues, server configurations, or security measures. Key causes include network connectivity problems, incorrect SMTP port usage, the sender's IP being blocked, temporary server unavailability, DNS misconfiguration, blacklist status, or SMTP server configuration problems.

Key opinions

  • Network Connectivity: A fundamental cause is often a network connectivity issue, preventing the sending server from reaching the receiving server.
  • Port and IP Restrictions: Incorrect SMTP port settings or the receiving server rejecting the sender's IP address can lead to this error.
  • Server Availability: Temporary server outages or unavailability on the recipient's side can prevent reading the SMTP banner.
  • DNS Misconfiguration: Incorrect DNS settings, preventing proper hostname resolution, can hinder connection establishment.
  • Blacklisting: The sender's IP address being blacklisted can cause connection rejections.
  • SMTP Configuration: Problems with the SMTP server's configuration, banner settings, or protocol version mismatches can trigger this error.

Key considerations

  • Check Firewall Rules: Ensure that firewall rules on both the sending and receiving servers allow SMTP traffic.
  • Verify DNS Settings: Confirm that DNS records are correctly configured to resolve the recipient's mail server.
  • Monitor Server Uptime: Consider server uptime and reliability, as temporary outages can cause this error.
  • Review Blacklist Status: Regularly check the sending server's IP address against known blacklists.
  • Validate SMTP Configuration: Ensure the SMTP server is correctly configured, including banner settings and protocol compliance.
  • Retry Connection: If the issue appears temporary, retrying the connection later may resolve the problem.
Marketer view

Email marketer from DigitalOcean responds by explaining the issue could be related to DNS misconfiguration, particularly if the sending server cannot resolve the receiving server's hostname to an IP address correctly.

September 2023 - DigitalOcean
Marketer view

Email marketer from MXToolbox responds suggesting that you check your server's IP address against blacklists, as being blacklisted can cause connection issues and the inability to read the SMTP banner.

February 2024 - MXToolbox
Marketer view

Email marketer from Reddit responds suggesting that the error might be caused by an incorrect port being used for the SMTP connection or the mail server not accepting connections from the sender's IP address.

July 2023 - Reddit
Marketer view

Email marketer from StackOverflow shares that this error often points to a network connectivity issue. They suggest checking firewall rules, DNS resolution, and ensuring the mail server is reachable from the sending server.

November 2021 - StackOverflow
Marketer view

Email marketer from SuperUser shares their experience that the error may indicate a problem with the SMTP server's configuration, possibly related to the banner settings themselves, or a mismatch in the expected protocol version.

February 2023 - SuperUser
Marketer view

Email marketer from Email on Acid explains that 'Unable to Read SMTP Banner' can arise from a sudden network outage or temporary unavailability of the receiving mail server. They advise retrying the connection later.

March 2023 - Email on Acid

What the experts say
6Expert opinions

The 'Unable to Read SMTP Banner, connection lost' error generally indicates a failure to establish a connection between servers. Experts suggest it means the servers were unable to understand each other during the initial connection, leading to the connection being dropped. Common causes include network failures, firewall restrictions, communication with a non-mail server, or issues with the receiving server's configuration. The error occurs before the HELO command, signifying a problem at a lower level. Retrying the connection is often recommended, along with checking server status and network connectivity.

Key opinions

  • Connection Failure: The primary issue is a failure to establish a proper connection between the sending and receiving mail servers.
  • Network Issues: Network failures, firewall restrictions, and incorrect server types are potential causes.
  • Pre-HELO Error: The error occurs before the HELO command is sent, indicating a low-level connection problem.
  • Transient Nature: The error may be transient, suggesting a temporary issue with the network or receiving server.
  • Configuration Problems: The receiving server's configuration might be the source of the problem.

Key considerations

  • Retry Connection: Retrying the connection after a short delay is a reasonable first step.
  • Check Network Connectivity: Verify that the sending server can reach the receiving server and that there are no network issues.
  • Review Firewall Settings: Ensure that firewall rules are not blocking the SMTP connection.
  • Monitor Server Status: Check the status of the receiving server to ensure it is operational and accepting connections.
  • Investigate Server Configuration: If the problem persists, investigate the receiving server's configuration for any issues.
Expert view

Expert from Word to the Wise Forum responds with 'Unable to read SMTP banner' usually points to a situation where the sending server could not properly establish a connection and receive the initial greeting from the receiving server, suggesting a firewall issue or server problem. Checking the receiving server's status and network connectivity is advised.

July 2021 - Word to the Wise Forum
Expert view

Experts from Email Geeks clarify the timing of the error. The banner is read before HELO, so 'Couldn't read banner' means HELO was not even sent. It's a failure outside of SMTP.

September 2022 - Email Geeks
Expert view

Expert from Email Geeks explains that "Unable to Read SMTP Banner, connection lost" likely means the server opened a connection but didn’t understand the other server during the connection, so it was dropped / closed / lost.

August 2022 - Email Geeks
Expert view

Expert from Email Geeks explains potential causes: Network failure, being firewalled off, or attempting to communicate with something that isn't a mail server. It can also happen to a smarthost that won't take 5xx for an answer.

October 2021 - Email Geeks
Expert view

Expert from Spamresource Forum explains that this error often indicates a transient network issue or a problem with the receiving server's configuration. They recommend retrying the connection after a short delay.

October 2024 - Spamresource Forum
Expert view

Expert from Email Geeks simplifies the explanation to: "network fall down. go boom."

May 2021 - Email Geeks

What the documentation says
4Technical articles

The error 'Unable to Read SMTP Banner' indicates a failure during the initial connection phase with the remote SMTP server. Documentation suggests this is often caused by network issues, firewall restrictions, or the remote server being unavailable or overloaded. Specifically, the sending server fails to receive the '220 Service Ready' greeting, meaning the TCP connection fails during the initial handshake, before any SMTP commands are exchanged.

Key findings

  • Connection Problem: Indicates a fundamental problem in establishing a connection with the remote server.
  • Network Issues: Network problems are a primary cause, including general network outages and congestion.
  • Firewall Restrictions: Firewalls blocking the connection can prevent the SMTP banner from being read.
  • Server Unavailability: The remote server might be unavailable, overloaded, or experiencing issues that prevent it from responding.
  • Initial Handshake Failure: The TCP connection fails during the initial handshake, before SMTP commands are exchanged, specifically the receipt of the '220 Service Ready' greeting.

Key considerations

  • Check Network: Verify network connectivity between the sending and receiving mail servers.
  • Examine Firewall Configuration: Ensure that firewalls are not blocking SMTP traffic on port 25 (or other configured SMTP port).
  • Verify Remote Server Status: Check the status of the remote server to ensure it is online and accepting connections.
  • Investigate Server Load: If the remote server is available, check its load and resource utilization to see if it is overloaded.
  • Review TCP Connection: Investigate TCP connection issues on the mail server.
Technical article

Documentation from RFC 5321, which defines the SMTP protocol, details the initial connection sequence. An 'Unable to Read SMTP Banner' error suggests a failure to receive the 220 Service Ready greeting from the server, indicating a problem before any SMTP commands are exchanged.

November 2021 - RFC Editor
Technical article

Documentation from cPanel explains that a connection timeout during the SMTP banner exchange can occur if the server is overloaded, experiencing network congestion, or if a firewall is blocking the connection. The 'Unable to Read SMTP Banner' message is a symptom of this timeout.

January 2022 - cPanel
Technical article

Documentation from Postfix mentions that the error suggests an issue with the TCP connection itself. It means that the mail server was unable to receive a response from the target server during the initial handshake.

December 2021 - Postfix
Technical article

Documentation from Exim Wiki explains that 'Unable to Read SMTP Banner' indicates a problem establishing a connection with the remote server. This can be due to network issues, firewall restrictions, or the remote server being unavailable.

August 2024 - Exim Wiki

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