Why are recipients exceeding their hourly email limit when BCC is used?

Key findings

• Recipient counting: Each address in the BCC field is typically counted as an individual recipient towards a mailbox provider's hourly or daily receiving limits. Even though BCC recipients are hidden from other recipients, they are still distinct delivery points that consume resources.
• Hidden impact: The primary challenge with BCC is that many ESPs may not display these recipients in their sending logs or attribute bounce codes directly to them, leading to confusion when bounce messages like 'recipient has exceeded their limit' are received for other recipients on the 'To' line.
• Receiver-side limits: Mailbox providers such as Microsoft 365, Gmail, Yahoo, and iCloud implement various rate limits to protect users from high volumes of email, including list bombing attacks or errant automated systems. These limits apply to the total number of messages a recipient can receive within a set period from all senders, not just one.
• Spam filtering: Sending a single email to a large number of BCC recipients can often trigger spam filters because it resembles bulk mailing, which is frequently associated with spam. This can lead to rejections or blocklisting, even if specific recipient limits aren't immediately exceeded.
• Domain reputation: Repeatedly hitting recipient limits can negatively affect your sender reputation, as it may signal problematic sending practices to mailbox providers. Understanding how BCC emails impact sender reputation is crucial.

Key considerations

• Verify ESP logging: Ensure your email service provider accurately logs all recipients, including BCC addresses, and provides clear bounce attribution. This visibility is vital for troubleshooting.
• Sender-side control: While recipient limits are set by the receiver, senders have control over their sending volume. Adjusting send rates to specific domains or recipients can help avoid these limits. See more on what 'rate limit exceeded' means.
• Alternative methods: For mass communications, consider using dedicated email marketing platforms or mailing list services rather than relying on BCC. These services are designed to handle bulk sends while adhering to deliverability best practices.
• Recipient communication: If you frequently send to specific corporate domains and encounter recipient limits, it might be beneficial to communicate with their IT or email administration to understand their specific receiving policies. Microsoft 365, for example, allows admins to configure custom recipient limits.

Key opinions

• Testing pitfalls: Marketers frequently see these limits hit when developers or testers rapidly send numerous emails to a single personal account or a small seedlist, quickly exceeding hourly quotas.
• Misleading bounces: A significant frustration is when an ESP's logs don't clearly show BCC recipients or misattribute bounces (like 'recipient limit exceeded') to 'To' line recipients, obscuring the true cause of the delivery failure.
• BCC as a culprit: There's a strong consensus that BCC is often the hidden factor behind unexpected rate limiting issues, particularly when organizational policies mandate its use for internal archiving or oversight.
• Provider consistency: The issue of recipients exceeding limits isn't unique to Microsoft; marketers report seeing similar bounce messages from major providers such as Gmail, Yahoo, iCloud, and Comcast, indicating a widespread mechanism to combat mail floods.

Key considerations

• Avoid BCC for bulk: Marketers strongly advise against using BCC for mass communications due to its deliverability implications and the lack of transparency in tracking. Better solutions exist for managing BCC'd emails for legal reasons.
• Internal education: Educate internal teams, including developers and sales, on the risks of sending high volumes to individual recipients or using BCC extensively, and the existence of these mailbox provider limits.
• Thorough troubleshooting: When encountering 'recipient limit exceeded' bounces, scrutinize all sending practices, including any hidden BCCs, and investigate whether the same recipient is being messaged multiple times or receiving from many different sources.
• Alternative sending methods: For transactional or commercial emails that need to reach many recipients, even internally, explore methods that provide better logging and respect recipient limits more gracefully than direct BCCing.

Key opinions

• Standard practice: Many mailbox providers, not just Microsoft, have long implemented recipient limits to prevent abuse and ensure service quality.
• Abuse prevention: These limits are primarily designed to counteract 'list bombing' scenarios, where an email address is maliciously subscribed to numerous services, overwhelming the recipient's inbox.
• Sender responsibility: The sender is ultimately responsible for managing their sending volume to avoid hitting recipient limits, even if the issue originates from the recipient's mail system settings.
• Beyond test accounts: While common in testing, exceeding recipient limits can also occur with legitimate transactional or commercial emails sent to multiple actual subscribers, highlighting the need for careful sending practices.

Key considerations

• Receiver-side settings: Acknowledge that per-hour message receive limits are often a configurable setting on the recipient's end, and they have sole control over how restrictive these settings are. This is a common aspect of how Microsoft rate limits email sends.
• Information sharing: Deliverability professionals should clearly communicate to customers that such limits are receiver-controlled. If the messages are valuable, the recipient's administrator might consider adjusting their settings.
• BCC transparency: Even though BCC recipients are hidden, they count towards recipient limits. This lack of transparency in logging from some ESPs can complicate troubleshooting, highlighting the inherent issues with using BCC for bulk communications.
• Reputation management: Hitting recipient limits, or being subjected to a blacklist or blocklist, can signal poor sending practices to mailbox providers, potentially leading to further deliverability issues.

Key findings

• Purpose of limits: Recipient limits are implemented to protect users from rapidly filling their inboxes with large numbers of messages from automated notification systems or mail storms, enhancing the efficiency and safety of email accounts.
• Recipient type inclusion: Documentation confirms that all recipient types, including BCC, count towards hourly and daily sending and receiving limits. Each address is treated as an independent delivery point.
• Specific error codes: Mailbox providers provide specific error codes, such as Microsoft's '5.2.122 - Recipient's per hour message receive limit exceeded', to clearly indicate when these thresholds have been breached. For more, review Microsoft's NDR documentation.
• Sender action: When limits are exceeded, documentation advises the automated mailer or sender to retry later and reduce the number of messages sent per hour to a specific recipient, highlighting a direct action for resolution.

Key considerations

• Adherence to guidelines: Senders should proactively consult the sending limits and guidelines of major mailbox providers (e.g., Google, Microsoft, Yahoo) to prevent hitting recipient limits, which can lead to emails being blocked or throttled.
• Impact on deliverability: Failure to manage sending volumes in accordance with recipient limits can lead to non-delivery reports and potential sending interruptions. This is closely related to resources restricted - exceeds recipients limit bounce messages.
• Administrative control: For organizational accounts, administrators often have the ability to configure or adjust these recipient limits, providing a potential avenue for resolving persistent issues for legitimate internal communications.
• Best practices for bulk: Official guidance encourages using appropriate bulk email sending methods, such as dedicated mailing services, rather than direct client-based BCCs, to manage high volumes and avoid triggering recipient limits.