[CentOS] DKIM Pass - Fail

Hi Dear Community Friends,

it is few days now, i am trying to figure out why DKIM is working / not
working. Any assistance would be very much appreciable.

Server IP is not blacklisted ever, MX, PTR SPF, DKIM records are available
in DNS. why it is working at Gmail, why failing at Yahoo?

Gmail
dkim=pass header.i=@digital-infotech.net

Yahoo:
domainkeys=neutral (no sig); from=digital-infotech.net; dkim=permerror
(future timestamp)

Prabh S. Mavi