Hey David, 
Thanks for reaching out. Please try the following. 
- Go to Settings->General 
 - If blank add 1.1.1.1 as primary and 8.8.8.8 as secondary DNS 
 - Save
 - Try to enable DKIM. 
 
- If there are already DNS servers entered in please remove the entries. 
 - Save
 - add 1.1.1.1 as primary and 8.8.8.8 as secondary DNS
 - Save
 - Try to enable DKIM. 
 
Let me know if this helps. 
Thanks, 
 
								Zach Sylvester
Software Developer
SmarterTools Inc.
www.smartertools.com