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Hello partner，
According to my investigation, this problem usually occurs in hybrids. do you have hybrid?
1.Please check this doc to see if it helps. Fix NDR error in Exchange Online
2.Also have you checked these two solved cases?
---554 5.4.12 SMTP; Hop count exceeded
---fixing the target address issue？
The recipient's domain being added to the relay also seems to cause the issue.
Regards
Shaofan
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