Hello Ionel,
I'm sorry that you're having issues with DKIM still and thank you for opening a ticket so we can look into this. We are not yet aware of what's preventing DKIM from signing messages. At the moment, we are aware of a workaround where you can remove the DKIM record, temporarily save the key size to something else, then change it back to the original value and re-enable DKIM. This should generate a new DKIM key that can be added to DNS and resolve the signature issue.
However, we understand this isn't an ideal method for anyone who is having this issue across a large number of domains. We will continue to look into this and will post updates here as they come.
Best Regards,
Zach Sylvester
Software Developer
SmarterTools Inc.
www.smartertools.com