We are currently evaluating this issue and will update this article when we have more information. In the meantime, create a copy of the email message and send it again. You may also want to check the groups in your email audience. If you have any groups that are configured to rebuild before sending, then make sure there are no problems with the group query. Here are some rebuild errors that could be contributing to this specific rebuild issue:
Error: The group list has cyclic dependencies !! - While attempting to send an email
Error: Group 'Group Name' (group_id=XXXXX) does not exist - When attempting to rebuild a query group
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