How failed deliveries and bounces are handled

When a forwarded email cannot reach your destination inbox, Maskr's privacy routing engine handles it in one of two ways: a temporary deferral or a hard bounce.

Temporary deferral (SMTP 451)

A 451 code means the routing engine is asking the sender's mail server to try again later. This happens when:

• Your account has hit the 200 emails/hour throttle
• A username subdomain is paused (see below)
• A transient network issue occurred

The sending server's outbound queue automatically retries, usually within minutes. You do not lose the email.

Hard bounce (Mailer Daemon message)

A 552 code means the message was permanently rejected. This happens when:

• Your Free/Starter bandwidth cap is fully exhausted and your wallet is empty
• The sender's address is on your mask's blocklist
• The mask is deleted

When a hard bounce occurs, the original sender receives a Delivery Status Notification from Maskr's mailer daemon explaining the reason. This is normal email behaviour and is not a security risk.

Why did the first email succeed but not the reply?

The most common cause is a tampered reverse-reply address. When you reply to a forwarded email, Maskr generates a cryptographically signed routing address. If the sender replies to that routing address again — rather than your original reply — the signature check fails and the email is bounced with reason code INVALID_ROUTING. Use your mail client's Reply button on the original forwarded email, not on Maskr's routing address.

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