Mail delivery uses your MX record. I am seeing:So both your MX records point to carrierzone.com. is this a mail service who then forward to you? Can they be set up with multiple IP's to deliver to?

If you are using MultiWAN, can't your Clearcenter DDNS be prioritised to your static connection then fail over to your dynamic then you just get your mail service to use your Clearcenter DDNS?