IMAP certificate is not valid and sent mails are going to Junk folder after enabling Mail Client Autoconfiguration

Hi everyone. Sorry if this is the wrong place to ask my question.

Here's the background:

• I have abc.net domain which is both fully quilifed hostname and virtual serverand , and abc.com which is second virtual server.
• Both abc.net and abc.com have Letsencrypt certificates
• Certificate of abc.net was copied to Postfix and ProFTPD (as they don't support multi domain SNI method)
• Certificate of abc.net and abc.com were copied to Dovecot for multi domain IMAP SSL usage
• I'm using mail.abc.net for mail clients to be used for abc.net and abc.com, since mail.abc.net is being used by Postfix SSL
• Mail Client Autoconfiguration enabled

Here are my questions:

1. When I configure either Thunderbird or Outlook, I get pop-up warning about IMAP certificate issue which points that I have invalid SSL certificate. Interestingly, CA information is all about auto generated imap.example.com (it's not an example, it shows actual domain like this) certificate and has no information about Let's Encrypt CA at all. But when I go to Virtualmin > Server Configuration > SSL Certificate, it shows me Dovecot (host abc.net) and Dovecot (host abc.com) At the end, I get IMAP certificate issue for both abc.net and abc.com.

2. I've changed Virtualmin > Email settings > Mail Client Configuration > Enable mail client autoconfiguration? to "Yes". Since then, all emails sent from either abc.net and abc.com go to the Junk folder of receiver. Removed autoconfig.cgi to test if the issue started to occur after this or not, and I can confirm that everything is okay when I have no autoconfig.cgi. But I need autoconfig, because I will have more domains like abcd.com, abcde.com and etc. which will use mail.abc.net as IMAP/SMTP address and without an autoconfig file I must manually configurate everything for end-user.

I hope it's clear enough. Looking forward to see replies and appreciate any comment