When you reply to an email, the majority of the time it reaches the recipient without a problem, but sometimes the email client in this case Google will seem to reject the message with the status of blocked - this can be a little confusing because you’ve replied to the previous email and that was absolutely fine no error?
Note : This is not a problem with gmail as any email platform set up for strict enforcement should perform the same actions, if you notice this is not being performed on other email platforms you might not have your security correct!
Usually, when replying to an email, the first message goes through without issue, but future replies in the same thread fail to be delivered. Gmail may show a permanent error, as if the recipient has blocked the message. At first glance, it might seem like a problem with your account—but the real issue often lies in how the recipient handles encrypted emails. Let’s explore why this happens.
The First Reply Works
When you reply to an initial email, Gmail signs the message with its DKIM signature. The recipient receives it as a valid message, and if their system supports it, the reply is processed normally.
At this stage, everything appears fine. The email is authenticated, correctly formatted, and in a format that the recipient’s system can handle.
Why Subsequent Replies Fail
Problems typically occur after the first reply. Many organizations now use proprietary encrypted email systems or S/MIME-like encryption solutions to protect sensitive communications. Evidence of this can often be seen in email headers, for example:
X-Forwarded-Encrypted: i=1; <encrypted token>@recipientdomain.com
Here’s what happens in these cases:
The first reply works because it is attached to an active encryption session. The recipient’s system can unwrap it and process it correctly.
Once the email thread is closed, archived, or the session expires, any subsequent replies are treated as invalid or improperly encrypted.
Gmail then attempts to send the reply, but the recipient’s server rejects it outright due to a mismatch in encryption metadata.
The result: Gmail reports a permanent delivery failure.
Why New Emails Go Through
Interestingly, composing a new email to the same recipient avoids the problem. Since there’s no reliance on an old encryption session, Gmail sends the message normally, and the recipient’s system can accept it.
How to Check if Encryption Is the Issue?
You can verify whether this is happening by inspecting the message headers of the emails you received. Look for lines like:
X-Forwarded-Encrypted: ...
or other encryption-related metadata (S/MIME, PGP, etc.). These headers indicate that the recipient’s system expects encryption, which can affect subsequent replies.
Key Takeaways
The problem is not with Gmail or your account—it’s a mismatch between Gmail and the recipient’s encrypted email system.
Replies to “closed” threads often fail due to encryption session timeouts or proprietary formatting requirements.
The simplest workaround is to start a new email rather than replying to an old thread.
If you communicate frequently with the recipient, consider asking if they can provide guidance on replying securely or bypassing encryption for trusted contacts.
Conclusions
Email encryption is excellent for protecting sensitive information, but it can make simple replies tricky. Checking message headers helps identify whether encryption is causing the delivery problem, and understanding how your email client interacts with the recipient’s system ensures your messages are delivered reliably.