DKIM of my domains are not verified.

Having DKIM is not a guarantee that emails do not go to junk. Does it explicitly say that DKIM validation failed and are there no DKIM headers in the email? And using test emails often cause them to be put into junk if the email doe snot look like a normal email. And Thunderbird has its own internal self-learning folder, so Sunderbird may put your Email into Junk, no matter if you use DKIM.

So the next steps are to check the mail headers; if they contain DKIM headers, then you know DKIM is set up correctly on your server. TThen check if is explicitly states that DKIM validation failed, if it says DKIM validation failed, then you probably have not set up the DKIM key correctly on your DNS server. A common problem is that you might have to remove the '' from within the key if you still use an older ISPConfig version which shows the key in split format that some other providers fail to parse, but that's all explained in detail in the ISPConfig email tutorial: https://www.howtoforge.com/ispconfig-email-account/

 

Howtoforge is not using DKIM, so that's perfectly fine that there are no headers in that email.

I posted a link that explains step by step how to enable DKIM.

I don't know that plugin.

No, DKIM works fine in ISPConfig. You just have to enable it and create a key as shown in the tutorial. and then publish that key in DNS to allow others to verify the DKIm signed email.

 

DKIM and MX Record are different topics. DKIM is a TXT record that holds the public key to allow others to verify signed emails, while the MX record tells other users where /under which hostname) to find your mail system.