I use rspamd.
This might be a little heavy if you’re solely looking for DMARC validation, but I use the other parts of rspamd as well for inbound email.
https://rspamd.com/modules/dmarc/
For a library, I mainly write Go and I use https://github.com/emersion/go-msgauth (formerly known as go-dkim).
Url seems to be https://docs.rspamd.com/modules/dmarc/
Thanks for the correction. I guess I had a mis-paste that stripped out the leading part of the hostname... weird.
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