There is a new problem that has arisen concerning all Red Hat external mailing lists that is related to a new DMARC policy that our security team has recently changed. As a result, the moderators of several Red Hat mailing lists (including me) have started seeing issues where posts to their mailing lists are being rejected as Undeliverable. There have been multiple internal support tickets filed in hopes of a resolution, but there is no workaround that I am aware of (except for some kind of mailing list configuration that needs to be done at the sender's site, which is unacceptable). Hopefully it will be fixed soon. If you receive such a response, please re-send it and cc: anderson(a)redhat.com. The response looks like this: us-smtp-1.mimecast.com rejected your message to the following email addresses: Discussion list for crash utility usage, maintenance and development (crash-utility(a)redhat.com) Your message couldn't be delivered. It appears that the email address you sent your message to wasn't found at the destination domain, or the recipient's mailbox is unavailable. The email address might be misspelled or it might not exist. Try to fix the problem by doing one or more of the following: Send the message again. Before you do, delete and retype the complete address. If your email program automatically suggests an address to use don't select it. Clear the recipient Auto-Complete List entry in your email program by following the steps in this article. Then resend the message, but before you do, delete and retype the complete address. If your email program suggests an address to use don't select it. Contact the recipient by some other means (by phone, for example) to confirm you're using the right address. Ask them if they've set up an email forwarding rule that could be forwarding your message to an incorrect address. If you're still unable to fix the problem, ask the recipient to tell their email admin about the problem, and give them the server that reported the error below. For Email Admins When Office 365 tried to send the message, the external email server returned the error below. This error was reported by an email server outside Office 365, and if the sender is unable to fix the problem by correcting the recipient's email address or clearing the Auto-Complete List entry, then it's likely a problem that only the recipient's email admin can fix. Check the error for information about where the problem is happening. For example, look for a domain name. The domain name will tell you which organization was responsible for the error. The recipient's email server could be causing the problem, or it could be due to a third-party service that your organization or the recipient's organization is using to process or filter email messages. If you can't fix the problem, contact the responsible party's email admin. This could be the recipient's email admin, your smart host service admin, or someone similar. Give them the error and the name of the server that reported the error to help them troubleshoot the issue. Unfortunately, Office 365 support is unlikely to be able to help with these kinds of externally reported errors. us-smtp-1.mimecast.com gave this error: Remote server returned unknown recipient or mailbox unavailable -> 550 Invalid Recipient - https://community.mimecast.com/docs/DOC-1369#550 [vVcOhPBXNFukJRNZ9UedSA.us264]