Why are emails bouncing from Telia and iCloud with a 4.3.2 error?

Key findings

• Temporary nature: A 4.3.2 error is a soft bounce, meaning the server expects the sender to retry delivery later. Unlike hard bounces, it doesn't mean the address is permanently invalid.
• Migration impact: Switching email service providers (ESPs) can trigger deliverability issues, as the new sending infrastructure (IP addresses, domains) lacks established reputation with major ISPs. This can lead to temporary blocks or rate limits, especially for high-volume sends.
• ISP sensitivity: Providers like Telia and iCloud (and other apple domains) are known for their stringent spam filtering and reputation requirements, making them quick to temporarily block traffic from unfamiliar or suspect sources.
• Quota issues: While 4.3.2 typically points to system issues, it can sometimes be related to a recipient's mailbox being full, preventing new messages from being accepted temporarily. See the IANA SMTP Enhanced Status Codes for more details.
• Sender reputation: Poor sender reputation (due to previous sending practices or a new, un-warmed IP/domain) can lead to ISPs temporarily rejecting messages to protect their users.

Key considerations

• Monitor bounce messages: Always analyze the full bounce message for more specific clues beyond just the 4.3.2 code. This often contains additional text from the recipient server.
• Warm up new IPs/domains: When migrating to a new system or IP address, implement a gradual IP/domain warming strategy. Start with small volumes to highly engaged recipients and slowly increase sending volume over time to build a positive reputation.
• Review authentication: Ensure that SPF, DKIM, and DMARC records are correctly configured for your new sending system. Misconfigurations can severely impact deliverability and lead to bounces. Our guide to DMARC, SPF, and DKIM can help.
• Check for blocklists: Verify if your sending IP or domain has been inadvertently placed on any major blocklists (blacklists), as this can lead to mass rejections from ISPs.
• List hygiene: Maintain a clean email list by regularly removing inactive or invalid addresses to prevent issues that can trigger ISP filters.

Key opinions

• Open rate drop as indicator: A sudden and significant drop in open rates, particularly for specific domains like Telia or iCloud, is often the first sign that emails are not reaching the inbox.
• Bounce messages are crucial: While open rates indicate a problem, obtaining actual bounce messages (like the 4.3.2 code) provides specific reasons and actionable insights for troubleshooting.
• System migration challenges: Migrating from one ESP (e.g., Campaign Monitor) to another (e.g., Salesforce) fundamentally changes the sending infrastructure, requiring a fresh start in terms of reputation building with receiving mail servers.
• Understanding soft bounces: Marketers understand that soft bounces, including 4.3.2 errors, are temporary but if persistent, they can point to underlying issues like rate limits or IP reputation problems. Our guide on soft bounces from iCloud offers more insight.
• ISP-specific behavior: Some ISPs, like Telia and iCloud, are known for their strict filtering rules, making them particularly sensitive to changes in sending patterns or unexpected traffic.

Key considerations

• Patience and persistence: Resolving deliverability issues, especially after a migration, often requires consistent monitoring and a structured approach to warming up and reputation building.
• Comparing systems: Comparing deliverability metrics between old and new systems helps pinpoint exactly where the issues started and which domains are most affected.
• Engage support: Don't hesitate to contact your new ESP's support team; they can often provide insights into their network's reputation and offer tailored advice for deliverability.
• Segment sending: During migration, consider segmenting your audience and sending smaller batches to highly engaged subscribers first to establish a good reputation. More on email bounce troubleshooting.
• Email list health: Ensure your email list is clean and verified to avoid sending to outdated or invalid addresses, which can quickly damage sender reputation and lead to more bounces.

Key opinions

• Reputation is paramount: A new sending system starts with a neutral, not positive, reputation. ISPs like Telia and iCloud will scrutinize traffic until trust is built, which can lead to initial rejections like 4.3.2. Learn about domain reputation.
• IP warming is non-negotiable: Attempting to send large volumes from a new IP without proper warming will almost certainly result in rejections, particularly by major inbox providers.
• Authentication alignment: Ensuring DMARC, SPF, and DKIM are flawlessly configured and aligned is critical. Any misstep can cause deliverability failures, as ISPs rely heavily on these signals. For instance, DMARC verification failures can lead to these issues.
• Throttling and rate limits: A 4.3.2 error can be an ISP's way of throttling traffic from a new sender. They allow a certain volume, then temporarily refuse more until trust is built.
• Feedback loops are key: Enroll new IPs in ISP feedback loops to monitor user complaints, which are vital for maintaining good sender reputation.

Key considerations

• Segment by engagement: When warming up, send only to your most engaged subscribers initially, as positive engagement signals improve your sender reputation much faster.
• Monitor blocklists: Proactively monitor your sending IPs and domains on major public blocklists. Instant notifications of a listing can allow for quick remediation. More on how email addresses end up on blacklists.
• Content quality: Even with perfect technical setup, poor content that triggers spam complaints will harm reputation. Ensure content is relevant and welcomed by recipients.
• ISP postmaster engagement: If systematic issues persist, reaching out to the postmaster teams of Telia and iCloud can provide direct insights and guidance on their specific policies and any perceived issues with your sending.
• Dedicated IP vs. Shared IP: Understand whether your new ESP uses dedicated or shared IPs. Shared IPs carry the risk of others' poor sending practices affecting your deliverability. Dedicated IPs offer more control but require diligent management.

Key findings

• SMTP standard definition: According to RFC 5321 (Simple Mail Transfer Protocol), a 4xx reply code indicates a transient negative completion reply, meaning the command was not accepted and the defined action did not occur, but the condition is temporary.
• Transient system error: The 4.3.2 error code typically translates to "System not accepting network messages" or "Server system error," indicating an internal issue on the recipient's mail server that temporarily prevents it from processing incoming mail.
• Retry mechanism: SMTP clients (your ESP) are expected to retry sending messages that result in 4xx errors, as these conditions are usually resolved within a few hours or days.
• Rate limiting implication: While not explicitly stated as a rate limit, a system not accepting messages can be an indirect result of an ISP's anti-abuse measures or traffic management when a sender exceeds an unestablished trust threshold.
• Compliance with best practices: ISPs (like Telia and iCloud) implicitly expect senders to adhere to email best practices, including proper authentication (SPF, DKIM, DMARC), low spam complaint rates, and gradual IP warming for new sending infrastructure.

Key considerations

• Adherence to RFCs: Ensuring your mail server's behavior conforms to SMTP RFCs helps minimize unexpected rejections from receiving servers.
• Configuring retries: Confirm that your ESP is configured to automatically retry sending 4xx bounce messages over a reasonable period (e.g., 24-72 hours) to allow the recipient server to recover.
• Postmaster sites: Consult specific postmaster pages (e.g., for iCloud Mail) for any publicly available guidelines or specific requirements that could lead to temporary rejections.
• Sender reputation data: Utilize available postmaster tools (e.g., Google Postmaster Tools for Gmail, though not specific to Telia/iCloud) to gain insight into your sending reputation and identify any issues impacting delivery.