I'm getting this error too.
Although all seems to be working for the mail server app on aapanel, when I send a mail to a test service like mailgenius, it says that the mail is not DKIM signed
Also this might be related
Since I have two domains in the mail server app, the "from" header of the emails is always the same (VPS hostname) although the email is being sent from one of those domains
As a side problem, whenever I add a valid SSL as mentioned in the official guide (making a SSL with a pan domain activated via DNS verification and then applying the same for the mail server app) for each domain on my mail server, I get the "add SSL" button as if I didn't added a valid SSL record, instead the expiry date as in others screenshots. How can I get over this?