Hey Brandon,
Thanks for reaching out to the community. This is a known issue that will be fixed in the next major release.
To work around this issue go to Settings->General and set the DNS servers to 1.1.1.1 and 8.8.8.8 or if they are already set to that make the fields blank then save. After doing this try to enable DKIM again.
After enabling DKIM you can safely reset the settings you had before if you wish.
Please let me know if this helps.
Kind Regards,
Zach Sylvester
Software Developer
SmarterTools Inc.
www.smartertools.com