How does using only BCC recipients affect email deliverability?

Key findings

• Suspicion magnet: Emails with only BCC recipients are highly scrutinized by spam filters and mailbox providers, as this is a common characteristic of spam.
• Deliverability impact: The lack of a 'To' field (or use of 'undisclosed recipients') can lead to emails bouncing or being delivered directly to the spam or junk folder, rather than the inbox.
• Technical issues: Some older mail clients may even crash if the 'To' field is completely empty. It can also appear similar to a DKIM replay attack, increasing scrutiny.
• Sender reputation risk: Consistent use of mass BCC can damage your sender reputation, making it harder for all your emails to reach the inbox in the future. Learn more about recovering domain reputation.

Key considerations

• Use an ESP for bulk sends: For sending emails to a large audience, an email service provider is the recommended solution. They handle individual sends, personalization, and compliance automatically, ensuring better deliverability. They are also much better for small group invites.
• Populate the 'To' field: If you must use BCC for a smaller, non-commercial list, put your own email address or a group alias (e.g., 'undisclosed recipients') in the 'To' field. However, be aware that 'undisclosed recipients' is an outdated practice and still carries some risk.
• Privacy vs. Deliverability: While BCC helps maintain recipient privacy, prioritize using proper email marketing tools to ensure your messages actually reach the inbox. The privacy benefit is lost if the email is blocked or sent to spam. This is important to ensure your email doesn't get put on a blacklist or blocklist.

Key opinions

• No need for BCC with ESPs: If you're using an email service provider, there's no technical reason to use BCC as they send emails individually to each recipient.
• Spam filter red flag: Sending emails with a large number of BCC recipients is considered a spammer's tactic and can trigger spam filters, leading to low deliverability rates.
• Privacy vs. delivery trade-off: While BCC offers privacy by hiding recipient addresses, this benefit is negated if the email consistently fails to reach the inbox.
• Domain and IP reputation: Frequent use of BCC for bulk sends can negatively impact your sender's domain and IP reputation, making future campaigns less effective. This can also affect your Google Postmaster Tools Domain Reputation.

Key considerations

• Avoid for commercial use: BCC should almost never be used for commercial or marketing emails. Instead, invest in a proper ESP for all your email outreach. For example, Influno states that emails with lots of BCC recipients can trigger spam filters.
• Personalized sends: ESPs allow for personalization of each email, even when sending to thousands of recipients. This improves engagement and deliverability significantly.
• Small, internal use only: BCC may be acceptable for very small, non-commercial, and internal communications where privacy is key and deliverability is less of a concern (e.g., a small team announcement). Even then, ensuring an address in the 'To' or 'From' field is advisable.

Key opinions

• Empty 'To' field is problematic: Not having any address in the 'To' field is generally bad for deliverability and can even cause some mail clients to malfunction. This impacts how your emails are delivered to the inbox.
• Spam resemblance: Commercial emails without the recipient in the 'To' field look spammy to both recipients and spam filters, reducing their likelihood of being delivered.
• DKIM replay attack similarity: The practice looks exactly like a DKIM replay attack, leading consumer mailbox providers to be much harsher on such emails. For more information, read our guide on a simple guide to DMARC, SPF, and DKIM.
• Mandatory 'To'/'CC' fields: Some individual and domain-level configurations require an address to be in the 'To' or 'CC' field for the email to reach its intended target, often as a routing or anti-spam measure.

Key considerations

• Avoid 'undisclosed recipients': While historically used, experts strongly recommend against using 'undisclosed recipients' in the 'To' header, as it's an obsolete format and can still trigger spam filters.
• Use ESPs for any bulk sending: For anything beyond internal, one-off communications, an ESP is critical. They handle personalization and compliance (e.g., unsubscription requests) which simple BCC cannot.
• Careful with SMTP relays: While technically possible to send BCC-only emails via an SMTP relay, experts advise extreme caution due to the significant deliverability risks involved.

Key findings

• RFC compliance: While RFC 5322 (Internet Message Format) specifies message headers, omitting the 'To' field can cause issues, even if technically permitted in certain contexts. RFC compliance often differs from practical deliverability.
• Spam filter logic: Email servers are designed to identify patterns indicative of spam. A message with only BCC recipients and no 'To' address often aligns with these patterns, triggering higher spam scores.
• Header parsing: Mail Transfer Agents (MTAs) parse headers rigorously. An unusual or missing 'To' header can lead to misinterpretation or outright rejection of the email.
• Authentication impact: While SPF and DKIM primarily authenticate the sending domain, the overall message structure, including headers, contributes to the email's legitimacy score. Anomalies can raise red flags during the authentication process, potentially leading to DMARC verification failures.

Key considerations

• Standard header practices: Even though BCC is a standard email field, the common and expected practice for valid bulk email is for the recipient's address to appear in the 'To' field for individual sends handled by an ESP. Using an ESP also allows for easier tracking of sent emails.
• MIME structure: The Multipurpose Internet Mail Extensions (MIME) standard defines the structure of email messages. While it supports BCC, the interaction of BCC-only with various MIME parts and mail clients can be unpredictable for deliverability.
• Evolution of anti-spam: Email systems have evolved sophisticated anti-spam mechanisms. Headers that deviate from typical, legitimate mass mailing patterns (which involve personalized 'To' fields) are increasingly scrutinized and penalized.