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Mail Sever ISP Load Balancing in Sonic Wall Firewall       NSA 250 M

I am facing a problem of failover in my Sonic Wall Firewall       NSA 250 M for mail server i am using two isp and also i configured that isp but if one isp is down then my server not working means external mails not receiving  how to solve this problem
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Technical Consultant
Ensure that there are two MX records. First MX record must point to 1st ISP's Public IP and second MX record must point to 2nd ISP's Public IP. Each one of them should have different priority. For example, first MX record priority 10 and second one priority 20. It should all then work if first ISP goes down. Also ensure that NAT and Firewall policies are configured correctly, in this case on the 2nd SonicWall because that's where there seem to be a problem.

You can also do SMTP telnet test to 2nd SonicWall from external network to ensure it is configured correctly. Let me know if you still have issue.