Some sent emails are dropped occasionally

Situation: The client has SBS running Exchange 2003. They find some sent emails never reach to the receipts occasionally. They also receive this message:

In the server Event Viewer, there is ID 2012 - SMTP could not connect to the DNS server 'xx.xxx.xx.xxx'. The protocol used was 'UDP'. It may be down or inaccessible.

Troubleshooting: Using nslookup command to test the DNS, we find the ISP DNS is not stable. We called the ISP regarding the DNS issue. It is confirmed that they have the DNS issue. Then they give us two new DNS and we re-assign these two DNS IP addresses in the DNS forwarders. That fixes the problem.

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