How to resolve increased soft and hard bounces after sending to a new segment with a new sender address?
Matthew Whittaker
Co-founder & CTO, Suped
Published 11 Jun 2025
Updated 16 Aug 2025
7 min read
Suddenly encountering a surge in soft and hard bounces after sending to a new segment with a new sender address can be a frustrating experience. This issue often signals deeper problems with your email program's configuration, list hygiene, or sender reputation. Understanding the specific error messages and their implications is the first step toward effective resolution.
When you introduce a new email segment, especially one from an acquisition, coupled with a sender address that isn't part of your primary domain, it can trigger a significant flag for mailbox providers. This change in sending patterns and identity can lead to increased scrutiny and, consequently, higher bounce rates. It's crucial to address these issues promptly to protect your overall email deliverability.
Understanding the sudden increase in bounces
Understanding what constitutes a hard bounce versus a soft bounce is fundamental. A soft bounce is typically a temporary delivery failure, like a full inbox or a server being temporarily unavailable. Mailbox providers often retry these sends. In contrast, a hard bounce signifies a permanent delivery failure, meaning the email address is invalid, non-existent, or has actively blocked your messages. The error message "envelope blocked - User Entry - mimecast SMTP error code 550" indicates a hard bounce, explicitly stating that the recipient (or their email system) has blocked you.
The message "rejected by header based manually blocked senders" further reinforces this. This isn't a temporary server glitch, but rather a deliberate block from the recipient's side, possibly due to a user manually adding your sending address or domain to a blocklist (or blacklist). These types of bounces directly impact your sender reputation and signal to internet service providers (ISPs) that your mail is unwanted.
Hard bounces vs. soft bounces
When an email returns with a hard bounce, it signals a permanent issue that cannot be resolved. These include invalid email addresses, non-existent domains, or explicit blocks by the recipient. You can learn more about this on Mailchimp's guide to soft versus hard bounces. These addresses should be immediately removed from your mailing lists to protect your sender reputation. Failing to do so can lead to further blocklisting and deliverability challenges.
If these hard bounce errors are appearing for your regular sender address to existing segments, it implies a broader impact on your sender reputation, which might be linked to the recent send. ISPs assess your reputation based on sending patterns, bounce rates, and user engagement. A sudden spike in hard bounces, even from a new segment or sender, can negatively influence how your established sending addresses are perceived.
The impact of a new sender and segment
Introducing a new sender address that isn't part of your primary, well-established domain can be problematic. Mailbox providers build trust with specific sending domains and IP addresses over time. When a new, unauthenticated, or lesser-known domain is introduced, especially with a large, unengaged list, it raises red flags. This is particularly true if the new segment consists of old or unverified email addresses that could be spam traps or invalid accounts.
The fact that the envelope address (the Return-Path or Mail-From address) is the same and shared with other senders is a critical piece of information. If one sender (or even one campaign from your own account) using that shared envelope domain generates a high volume of complaints, hard bounces, or spam reports, it can negatively affect the reputation of the entire envelope domain, including all other senders using it. This is why issues from a new, problematic segment could propagate and impact your existing, healthy segments.
New segment and sender
Unverified contacts: Sending to an uncleaned list from an acquired company can contain a high percentage of invalid or inactive email addresses. This directly contributes to high hard bounce rates.
New sending identity: Using a sender address not aligned with your primary domain, especially without proper warming, signals suspicious activity to ISPs.
Changing the sender address to a personal one under the same domain as your primary, recognized address might offer a temporary improvement for that specific recipient, but it doesn't solve the underlying issue of why recipients are blocking you. If the block is at the domain level (e.g., @yourdomain.com), changing the local part of the email address (e.g., from marketing to personal) won't bypass the block. Focus on addressing the cause of the blocks themselves.
Steps to mitigate and recover
The first and most critical step is to clean your email list rigorously. Remove all hard bounced addresses immediately. These recipients explicitly do not want your mail, and continued attempts will only further damage your sender reputation. For any new segments, especially acquired ones, a thorough validation process is essential before sending. This helps identify and eliminate invalid email addresses, honeypots, and known complainers, reducing your bounce rate and improving deliverability.
Ensure that your sender domain is properly authenticated with SPF, DKIM, and DMARC. These protocols verify your sending identity and help mailbox providers trust your emails. If the new sender address's domain (or the new segment's primary domain) isn't fully authenticated, it's more likely to be flagged as suspicious. A strong DMARC policy (p=quarantine or p=reject) with proper alignment can significantly improve trust, but it requires careful implementation.
For new segments, especially those with unknown engagement levels, implement a gradual email warming strategy. Start with smaller volumes to the most engaged contacts within that segment and slowly increase volume over time. This helps build a positive sending history with mailbox providers for the new list. If your envelope address is shared, investigate if you can configure a custom envelope domain (Return-Path) specifically for your mail. This isolates your sending reputation from other users sharing the main envelope, giving you more control over your deliverability.
Regularly monitor your bounce rates, complaint rates, and sender reputation metrics. Tools like SendLayer's guide to fixing bounce rates emphasize this importance. Pay close attention to blocklist (or blacklist) monitoring to quickly identify if your IP or domain gets listed. This proactive approach helps you detect issues early and take corrective action before they severely impact your deliverability across all segments.
Proactive measures for future sends
Moving forward, prioritize list quality over quantity. Implement robust consent mechanisms, like double opt-in, for all new subscribers. This ensures that recipients genuinely want to receive your emails, which reduces the likelihood of complaints and blocks. Regularly re-engage or remove inactive subscribers to maintain a healthy and responsive list.
Segment your lists effectively. Sending targeted content to smaller, engaged segments can yield better results than blasting to a large, unsegmented list. This practice not only improves engagement metrics (opens, clicks) but also minimizes the risk of recipient fatigue and subsequent blocks. Focus on providing value with every email to foster a positive relationship with your subscribers.
Finally, review your email content and sending practices. Ensure your emails are relevant, engaging, and provide clear unsubscribe options. Avoid spammy keywords, excessive images, or poor formatting. A positive user experience reduces complaints and helps maintain a good sender reputation, preventing future bounce spikes.
Views from the trenches
Best practices
Always validate new email lists, especially those from acquisitions, before sending any campaigns to them.
Ensure your sender domain is fully authenticated with SPF, DKIM, and DMARC for all sending addresses.
Implement a gradual warming strategy for any new sending IP or domain, starting with highly engaged contacts.
Regularly monitor your bounce rates and DMARC reports to catch deliverability issues early.
Common pitfalls
Sending to an unverified acquired list, leading to a high volume of invalid addresses and spam traps.
Using a new sender address or domain without proper authentication and warming.
Ignoring hard bounce messages, which signals continued sending to invalid or unwanted addresses.
Failing to use a custom envelope address, leading to shared reputation issues across senders.
Expert tips
Use a custom envelope domain to isolate your sending reputation from other senders, even on shared IPs.
Investigate past unsubscribe attempts for contacts generating explicit blocks.
Segment new lists and prioritize sending to the most engaged users first to build trust.
If using an ESP, understand their bounce management policies and how they handle shared IP/domain reputation.
Marketer view
Marketer from Email Geeks says that sending to a new segment with a different sender address can initially improve inboxing, but recipients might eventually block the new address too. It is better to remove contacts that hard bounced immediately.
2022-02-02 - Email Geeks
Expert view
Expert from Email Geeks says that an 'envelope blocked - User Entry' error is not a soft bounce, but a hard bounce, indicating the user has explicitly blocked your email. These contacts should be removed from all future sends.
2022-02-02 - Email Geeks
Restoring your email deliverability
Resolving increased soft and hard bounces, especially after introducing new segments and sender addresses, requires a multifaceted approach. It involves a deep dive into your list hygiene, ensuring proper sender authentication, and employing strategic warming for new sending profiles. By systematically addressing these areas, you can recover your sender reputation and ensure your emails consistently reach the inbox.
