The A record error is correct as an A record should only hold the IP adress not the domain name, you can try with CNAME if you don't want to enter the IP for A record, one more thing setting up mailserver is really hard to begin with, like setting up DNS records exactly as you configure it with your mail server config including DKIM, DMARC, SPF and MX everything should be accordance to your mail server configuration. I am not using mail server with aapanel for one or more reasons i prefer the mails to be handled by zoho or biz.mail.ru, they are easy, free and works stupidly awesome the only drawback would be you not being to able to send mail from php using sendmail() function but the SMTP works great for me with sparkpost and others. If you need more clarification feel free to pm i will respond as soon as possible.
