What causes a Symantec email block and how to fix it?

Key findings

• Misconfigured MX records: An incorrect setup of MX records can lead to your mail server attempting to relay through an intermediate service like MessageLabs, even when the final destination is another provider such as Office 365. This often results in a 553 relay access denied error.
• Authentication failures: Symantec (Broadcom) extensively uses email authentication protocols like DMARC, SPF, and DKIM. Failures in these checks can lead to emails being blocked or sent to junk, as detailed by some deliverability experts. Proper configuration of these records is crucial.
• Sender reputation: Even with correct technical setup, poor sender reputation, often influenced by high spam complaint rates or sending to invalid addresses, can trigger Symantec's blocklists. This can lead to your emails being blacklisted or blocklisted. Understanding what it means to be blacklisted is the first step.
• Content filtering: Emails containing spam content, suspicious URLs, or certain attachment types may be blocked by Symantec's advanced filtering systems, regardless of the sender's reputation.

Key considerations

• DNS checks: Verify your domain's MX records to ensure they correctly point to your intended mail gateway (e.g., Office 365) and that Symantec's servers are not mistakenly being used as an open relay.
• Sender configuration: Confirm that your mail server is configured to send emails directly to the intended recipient's MX server, rather than attempting to relay through Symantec's infrastructure if it's not the primary entry point for the recipient's domain.
• Deliverability monitoring: Regularly monitor your email deliverability and sender reputation to proactively identify and address issues that could lead to blocks by major email security providers.
• Content best practices: Adhere to email marketing best practices, avoiding spammy content, excessive links, or suspicious attachments, to prevent triggering content-based filters.

Key opinions

• Diagnostic challenges: Marketers frequently find themselves troubleshooting blindly, particularly when lacking direct access to mail server configurations or detailed bounce reports. This makes identifying the exact cause of a Symantec block difficult.
• MX record impact: Concerns arise when the email sender appears not to be honoring DNS priority routes, leading to emails being sent to unintended mail exchange servers like Symantec's when they should go elsewhere (e.g., Office 365). Troubleshooting Outlook email issues often involves checking MX records.
• Spam content sensitivity: Email security systems like MessageLabs are known to block emails containing any form of spam content, including forwarded spam or URLs associated with spamvertisements. This is a common topic in community forums.
• Client-side blocks: Sometimes, Symantec blocks are related to local site misconfigurations or firewall settings on the recipient's side, which prevent email clients from properly receiving messages or attachments.

Key considerations

• Verify DNS records: It is critical to regularly check and validate MX records to ensure emails are routing through the correct gateways, especially when using third-party filtering services or cloud email providers. Incorrect MX priority can easily lead to blocks.
• Review email content: Before sending, scan your email content for potential spam triggers, including suspicious keywords, too many links, or certain file attachments that Symantec might flag. This can help prevent your emails from going to spam.
• Check local configurations: If recipients report issues specifically with Symantec deleting or blocking attachments, advise them to check their local firewall and program control settings to ensure email clients are not being unduly restricted.
• Understand relay attempts: Be aware that a 553 error often means your server is trying to use Symantec's servers as an unauthorized relay for a domain they do not manage or are not meant to process, requiring adjustment of your outgoing mail server settings.

Key opinions

• Local site misconfiguration: If you're not intentionally sending email through Symantec's servers as a relay, then a block is almost certainly due to a local site misconfiguration. This means your mail server is attempting an unauthorized relay.
• Borked MX arrays: Experts note that MX arrays can be configured incorrectly, such as when Office 365 is publicly visible as a primary MX record while MessageLabs is also present. Ideally, filtering services like MessageLabs should sit in front of the final mail destination.
• MX weight irrelevance: The MX weight (priority) in DNS records often means little in practice, as many mail servers do not strictly honor it. This can lead to unexpected routing and blocklists.
• Spam trap history: Historically, high-preference MX records (those with higher numerical values, meaning lower priority for legitimate mail) were sometimes used as backup MXes with vaguer spam filters. This made them targets for spammers and good locations for DNSBLs (DNS-based Blackhole Lists) and honeypots.

Key considerations

• Prioritize proper mail flow: Ensure your MX records and mail server configurations align with the intended mail flow, particularly if using a filtering service like MessageLabs. It should typically be the first hop for inbound mail, not a relay for outbound.
• Review DNS for misconfigurations: Thoroughly inspect your domain's DNS records, especially MX entries, to ensure they do not unintentionally expose internal servers or create unauthorized relay paths. This includes looking for causes during an email blacklisting.
• Understand SMTP errors: When encountering SMTP 553 relay errors, recognize that they are a strong indicator of an unauthorized relay attempt, directing your troubleshooting efforts to your mail server's configuration rather than recipient-side filtering.
• Consult Symantec documentation: For specific Symantec block messages, refer to the Symanteccloud troubleshooting guide for detailed error message explanations and resolution steps directly from the source.

Key findings

• SMTP 553 relay errors: Symantec's troubleshooting guides, such as the one found at symanteccloud.com/troubleshooting, indicate that a 553 error (e.g., #5.7.1) signifies an attempt to use their servers as an unauthorized relay. This implies the sender's mail server is not configured correctly for the intended mail flow.
• DNS configuration requirements: Documentation emphasizes the necessity of accurate MX records and proper SPF/DKIM authentication to ensure emails are routed and validated correctly. Misconfigured DNS entries can lead to rejection as the system may perceive an incorrect or suspicious mail path. An in-depth guide to email blocklists often includes these details.
• Security policy enforcement: Symantec's systems are designed to strictly enforce anti-spam and anti-malware policies. Any mail flow that deviates from established security practices or appears suspicious will be blocked to protect recipients.

Key considerations

• Review your mail server settings: Ensure your mail server is configured to send directly to the recipient's primary MX record (e.g., Office 365) if Symantec (MessageLabs) is not the intended relay or filtering service for your outgoing mail. Do not attempt to use Symantec's inbound gateways for outgoing mail unless explicitly authorized.
• Validate DNS MX records: Confirm that your domain's MX records accurately reflect your intended mail routing. If you are using a third-party filter like MessageLabs for inbound mail, ensure your outbound mail flow does not inadvertently try to relay through it when sending to domains it does not host.
• Implement DMARC correctly: Ensure your DMARC, SPF, and DKIM records are properly set up and aligned. Correcting DMARC issues is vital for major providers, as authentication failures are a significant cause of blocks.
• Review bounce messages: Pay close attention to the specific error codes and messages provided in bounce notifications, as they often contain links to official troubleshooting resources that can guide you to the exact solution.