Hello @mara2021 ,
Welcome to our forum!
Based on your description, I would like to confirm when you started experiencing this problem. Have you had a chance to test whether this problem also occurs when sending to multiple recipients in other domains?
If you are able to send mail to other domains, then the problem may be caused by the recipient and not by you.Besides,You mentioned that you ran get-messagetrace but could not find out whether the email was delivered, failed or pending. Could you please provide detailed information about what is the exact situation?
Additionally, the error message "400 4.4.7 Mail Delayed" usually means that the recipient's mail server is experiencing a temporary problem and is unable to accept the message at this time. This can happen for many reasons, such as network congestion, large amounts of incoming mail, etc. it could be that their servers are throttling you, so the first suggestion is to wait and try again.
Another possible reason is that the MX record of the receiving domain cannot be resolved correctly,it is recommend that you try to check this with the nslookup command,also make sure that your server's IP address is not blacklisted on the receiving side.
As for your reasons for thinking about sending too many messages at once, sending a large number of messages in a short period of time may trigger spam filters or other security measures on the recipient's mail server, which may cause messages to be delayed or blocked,however, this will usually result in a different error message.
Hope the above can be helpful.
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