So far, we paid little attention to email delivery issues. We knew delivering to rediffmail is a pain. So we discouraged our users from using rediffmail. Apart from that we had FCrDNS and SPF configured and working fine. We had also configured DKIM. And then a month ago, we also added DMARC in monitor mode.
This is a companion discussion topic for the original entry at https://www.elitmus.com/blog/technology/importance-of-date-field-in-an-emails-header/