This is a Microsoft Exchange Server-specific SMTP status response that occurs when the recipient's Exchange Server mail queue has been stopped. The mail queue can often be stopped due to too many open connections to the server or while the network administrator is troubleshooting an issue.
Fixing this issue might be difficult from your end since it is hard to determine the exact reason. For that, you need to contact the network administrator.
Meanwhile, you can check your computer's local internet connection, evaluate any probable server network difficulties, or check for any local network issues.
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