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Fixing the 550 5.7.1 [BBD] error

Spammers often have dirty mailing lists, containing large numbers of invalid recipient addresses. If MailChannels detects that a sender has been sending to large numbers of invalid recipients recently, the sender may be blocked with the [BBD] error code.

What can I do to unblock the sender?

If the sender is delivering messages to a mailing list, please verify that the addresses in the list are valid. You can use a tool such as or for address verification.

Generally speaking, to avoid the [BBD] error, senders should

  1. Reduce the number of invalid recipients on their mailing lists by using a confirmed opt-in process, consistent with M3AAWG Sender Best Practices.
  2. Remove invalid recipients from their lists immediately.
  3. Only send to subscribers who have opted in to their lists.

MailChannels automatically stops generating the [BBD] error when the frequency of invalid recipients falls to a normal level. Note that we do not use a specific threshold of invalid recipients, and cannot therefore advise on a specific permissible rate of invalid recipients.

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