Is there no way to create an MX record with a static IP address that is different than the Dynamic IP that I am using for everything else?
My situation is that I have both a high speed dynamic internet service that is being used for several websites, and a DSL with a static IP that I use for my personal mail server. I need to create a MX record that will point to the static IP (since mail on dynamic IPs does not work out well).

There is really no way to create an MX record pointing to a static IP address without actually having a static IP address to utilize available. We do offer email store/forward service which will allow you to use to receive incoming emails and utilize Dynu MX records. There is also SMTP outbound service which allows you to send outbound emails via our SMTP server if you are having issues sending out emails but are still able to receive emails.

https://www.dynu.com/en-US/Email

Note: The issue with ISP provided static IP is that the reverse DNS still does not reflect the domain name for which email is hosted. You can try to contact your ISP and see if they can update the reverse DNS for the IP address provided to you with your domain name and that can be quite helpful. You can also setup SPF record as well as DKIM to make the emails more legitimate.

If you have a static IP address that your email server is running on, you can create an A record for mx1.yourdomain.com to point to that static IP address. Then you can add an MX record mx1.yourdomain.com. Remember to add reverse DNS for the static IP address to point to mx1.yourdomain.com as well.