I sent an email to two friends on Tuesday using Windows Live Mail. They both received it but I received a mail delivery system response saying it couldn't be delivered to firstname.lastname@example.org, mailbox disabled (554.30). This is an address I have never heard of and certainly did not send to. It hasn't happened since. Any advice would be welcome.
Have you tried sending to the same two individuals since?
The most obvious thought is that one of them has auto forwarding set up on the account. Because of the way this works the mail is sent as is, with the same envelope sender address. That means any bounces will come back to you.
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Thank you for making your first post regarding your outgoing mail being copied to another address. I am glad to hear that the issue has been pinned down and you have informed the person thanks to @ravenstar68's guidance.
If there's anything else we can help with, please do let us know 🙂