How does using only BCC recipients affect email deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 16 May 2025
Updated 16 Aug 2025
7 min read
Blind Carbon Copy, or BCC, is a common feature in email clients that allows you to send a message to multiple recipients while keeping their email addresses hidden from one another. While it serves a valid purpose for privacy, especially in certain contexts, its use, particularly when emails are sent with only BCC recipients and an empty 'To' field, can significantly impact email deliverability. This practice is often viewed with suspicion by mail servers and spam filters.
Many email users opt for BCC to avoid exposing recipient lists, which can prevent issues like reply-all storms or unauthorized collection of email addresses. However, what seems like a simple privacy measure can inadvertently trigger spam filters, leading to your messages being blocked or routed directly to junk folders. This often stems from how email systems process messages without a visible recipient in the 'To' or 'CC' fields.
The core issue lies in the design of email protocols and how modern spam detection mechanisms operate. Email deliverability isn't just about sending an email, it's about ensuring it reaches the intended inbox without being flagged as suspicious. Understanding these nuances is crucial for anyone relying on email for communication, whether for personal updates or business outreach.
The technical implications of an empty 'to' field
Sending an email with an empty 'To' field and only recipients in the BCC field is a red flag for many mail servers and spam filters. The 'To' field is a fundamental part of an email header, signaling the primary recipient of the message. When this field is absent or contains a generic placeholder like undisclosed-recipients: ;, it disrupts the expected structure of a legitimate email. This can even cause some older mail clients to crash.
This practice is often associated with bulk email sending by spammers who aim to hide recipient lists. Consequently, automated spam detection systems are configured to scrutinize, and often reject, emails that deviate from standard email formatting, especially those lacking a clear 'To' address. It can appear to be a form of email authentication or DKIM replay attack, making mailbox providers harsher in their filtering.
Example of an obsolete header
undisclosed-recipients: ;
The underlying technical reason involves how mail transfer agents (MTAs) and mail clients interpret and route messages. While BCC hides recipients from each other, the mail server still processes each recipient individually. If the 'To' field is missing, it creates an unusual header structure that can raise flags during the initial server-to-server communication, leading to increased scrutiny and a higher likelihood of the email being marked as spam or even blocked outright.
Impact on spam filters and sender reputation
The impact on your sender reputation and email deliverability when using only BCC recipients can be significant. When emails are sent this way, they often bypass standard personalization, which is a key factor in building positive sender reputation. Internet Service Providers (ISPs) and mailbox providers increasingly use sophisticated algorithms to detect spam and protect their users. An email sent solely to BCC recipients, especially in large volumes, lacks transparency and often appears to be a mass mailing tactic used by spammers.
When an email server receives a message with many hidden recipients, it scrutinizes it more rigorously than emails sent to visible recipients. This heightened scrutiny can lead to your emails being flagged as suspicious, which in turn degrades your sender reputation. A poor sender reputation can result in future emails from your domain being sent to spam folders, being delayed, or even being outright rejected.
Repeatedly sending emails in this manner, particularly if they generate spam complaints, can lead to your IP address or domain being added to a blacklist (or blocklist). Once on a blacklist, your email deliverability can suffer severely across multiple mailbox providers. Recovering from a blocklist can be a lengthy and challenging process, requiring significant effort to rebuild your email domain reputation. It's a delicate balance, and abusing the BCC function can easily tip it into negative territory.
Best practices for sending to multiple recipients
For mass email communication, especially commercial or marketing emails, relying solely on BCC is almost always a poor choice for deliverability. The ideal solution is to use an Email Service Provider (ESP). ESPs are designed to handle bulk sending efficiently and safely, ensuring each recipient receives a personalized email without exposing other addresses. They also manage critical aspects like email authentication, unsubscribe requests, and bounce handling, which are crucial for maintaining a healthy sender reputation.
If you are sending to a small, non-commercial group (e.g., family or close friends for a one-off event invitation) and privacy is paramount, a common workaround is to place your own email address in the 'To' field and all other recipients in the BCC field. This provides a visible 'To' address, which can help mitigate some of the spam filter issues, while still keeping the recipient list private. However, this method is not scalable and is not recommended for professional or regular communications.
BCC only for mass emails
High risk of being flagged as spam due to missing 'To' header and bulk sending patterns.
Damages sender reputation, leading to lower inbox placement over time.
Lacks personalization and proper unsubscribe mechanisms, non-compliant with regulations.
Using an Email Service Provider
Optimized for high-volume sending with individual emails, improving inbox placement.
Helps maintain a strong sender reputation through proper authentication and bounce management.
Enables personalization, segmentation, and automated unsubscribe processes, ensuring compliance.
Ultimately, for any significant volume of email, especially if it's external or commercial, leveraging an ESP is the recommended best practice. This not only enhances deliverability but also provides valuable analytics and compliance features that a manual BCC approach cannot. If you're wondering, is BCC better than an ESP for small group invites, the answer typically leans towards an ESP for optimal results, even then.
Navigating BCC for deliverability
The practice of sending emails with an empty 'To' field and only BCC recipients is a carryover from a time when email protocols and spam filters were far less sophisticated. Today, it presents significant deliverability challenges, risking your messages being flagged as spam and your sender reputation being damaged. For mass emails, leveraging an ESP is the best solution, ensuring proper personalization, compliance, and optimal inbox placement. For very small, non-commercial groups, placing your own address in the 'To' field can be a temporary workaround, but it doesn't scale.
Prioritizing good email hygiene, including proper recipient management and adhering to email sending best practices, is crucial for consistent deliverability. This means avoiding shortcuts like mass BCCing, which can lead to your emails being added to a blocklist or blacklist, and instead investing in robust sending methods that respect both recipient privacy and email ecosystem norms.
Always remember that successful email delivery relies on trust and adherence to established standards. Anything that deviates from these standards, such as an unusual 'To' field or a high volume of hidden recipients, will likely be scrutinized and potentially blocked by email providers focused on combating spam.
Views from the trenches
Best practices
For mass emails, always use an Email Service Provider (ESP) to ensure individual recipient addresses are handled correctly and to leverage features like personalization and unsubscribe management.
If sending to a small, non-commercial group where privacy is essential, consider placing your own email address in the 'To' field and all other recipients in the BCC field.
Implement robust email authentication protocols such as SPF, DKIM, and DMARC to build and maintain a strong sender reputation and signal legitimacy to mail servers.
Regularly monitor your domain and IP for inclusion on email blocklists (or blacklists) to quickly identify and address any deliverability issues.
Segment your audience and personalize emails to each recipient, as this significantly reduces the likelihood of messages being flagged as spam.
Common pitfalls
Sending emails with an empty 'To' field, which is often seen as suspicious by spam filters and can lead to immediate rejection or junk folder placement.
Using BCC for large-scale marketing or commercial campaigns, as this practice closely resembles spamming tactics and damages sender reputation.
Failing to include proper unsubscribe options when using BCC, leading to higher spam complaints and potential blocklisting (or blacklisting).
Ignoring the technical implications of BCC on mail server processing, which can result in unexpected delivery failures or mail client issues for recipients.
Assuming that hiding recipients via BCC completely protects privacy without considering the broader impact on email deliverability.
Expert tips
Some older mail clients can crash when receiving emails without a 'To' field. Ensure compatibility if your audience uses diverse clients.
The absence of a 'To' field can be misinterpreted as a DKIM replay attack by modern spam filters, leading to stricter filtering.
Many domain-level configurations and individual mail servers require an address in the 'To' or 'CC' field for emails to be delivered.
Mass BCCing is often a 'spammer trick' and can trigger automated spam detection systems, regardless of content.
Avoid relying on 'undisclosed recipients' in the 'To' header, as it's an obsolete format and can still negatively impact deliverability.
Expert view
Expert from Email Geeks says that not having a 'To' field at all is problematic, as some mail clients might crash. Putting your own email address in the 'To' field is a better practice.
2023-07-05 - Email Geeks
Expert view
Expert from Email Geeks notes that commercial emails lacking a recipient in the 'To' field appear spammy to both recipients and spam filters. This also looks exactly like a DKIM replay attack, leading to stricter filtering by consumer mailbox providers.
