What if SMTP server log shows these errors and mail is marked as SPAM?
Tainted filename '/etc/virtual/*.com/dkim.private.key'
unable to open file for reading: /etc/virtual/*.com/dkim.private.key
I have seen this in DirectAdmin account mail accounts SMTP outgoing mail log.

What helped was to check DNS section in my hosting account and make sure the DKIM TXT record there equals to the DNS record in my cloudflare.com DNS section. Especially the DKIM selector part: abc._domainkey

I let it some hours to propagate.