sohopros
Hi,
If your mail server already handles a DKIM signature or if there is a way to enable this DKIM signature on your server without using the AcyMailing one, then you should NOT use the AcyMailing DKIM signature but configure the one you have on your server.
Cpanel, Plesk, Sendmail, Postfix, QMail will enable you to handle a DKIM signature automatically.
The same way, if you already use an SMTP server which handles DKIM, you should not enable the AcyMailing DKIM signature.