Hi,
According to your description, I understand that external user failed send message to DG with error SMTP Error 451 4.7.0 Temporary server error. Please try again later. PRX1.
If I misunderstand your concern, please do not hesitate to let me know.
I want to double confirm whether all external account cannot send message to your environment, or this external user can send message to a single user.
If all inbound mail flow failed, it indicate that inbound mail flow failed. Please refer to
Mail flow.
If this issue only occur on this DG, please try suggestion as Sathish provided.
Please post the result, its more helpful for further troubleshooting.
For error SMTP Error 451 4.7.0 Temporary server error. Please try again later. PRX1, it indicate that the encoded SMTP response for when proxy fails because destinations were not provided.
Meanwhile, please use Protocol log and Message tacking log to get more details about this issue.