ISPConfig 3 mail server SPAM question

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I have a working configuration of ISPConfig 3.0.16 with mail server and website/virtual domain handling.

All my domains are FQDN and the server send and received mails using webmail as well as IMAP/SMTP mail clients.

My users have reported that their email is detected as spam to many of their recipients.

The question is: How to make my server legit and known as NOT spam for every spam detection engine/website out there?

Do I have to complete any kind of forms or subscribe to a specific organisation?

Please help

Thanks