Your message wasn't delivered because the recipient's email provider rejected it

Does it only happen to one sender, or everyone gets rejected?

I don't think that bounce email sender received is complete, usually there's more information in that email than what you shared. If you don't mind, ask sender to send you the complete bounce email, especially if it contains error code or rejection reason code, etc.

你是回復回信嗎？
檢查你寄出郵件跟回復回信郵件的 Message-id 是否重複，這是從 Outlook 2013 以來的 bug ，滿隨機的，一直找不到原因。

如果 Message-id 一樣，直接開一封新郵件，把你的回復回信貼上去寄出。

RFC 規範是禁止 Message-id 相同。

No boucing email for me.

only sender received an email said :

"Delivery has failed to these recipients or groups: ******@outlook.com Your message wasn't delivered because the recipient's email provider rejected it."

it looks like the email is being rejected and i wasnt aware of it until the sender told me it is rejected.

Hello

Is that the complete bounce back email? If not, can you please share the bounce email? It contains important diagnostic data that helps shed some light of the root cause. Feel free to redact personal information when you share.