Email delivery issues lately

We are running an IBM Series I v7r4m0, with Domino 12.01FP1, we host our
own pair of external DNS servers within our DMZ network (just an aside)
Lately, when sending emails to gmail and a couple of other domains, in the
last 2 weeks, they are getting bounce backs.

Our DNS Records include a google verification TXT record and an SPF
record.

When the emails bounce back from gmail, and it is not all of them, its
very random, they point to our not being "authenticated" via SPF or DKIM
or DMARC

I did some searching on HCL and I am seeing the DKIM is now available for
Linux/Win64 versions of Domino, however, nothing points to this happening
for the Series I systems.

I guess my question is, have any of you experienced these types of
bounce-backs recently? Have you any ideas for us?

Thanks, for any ideas!

Carol Taylor