I only found out a few days ago that Gmail and Yahoo (and possibly others) will require senders to have SPF, DKIM and DMARC records (as well as certain other criteria) from February 2024: https://postmarkapp.com/blog/2024-gmail-yahoo-email-requirements
I'm currently setting this up for a couple of my domains (it's already working on a test sub-domain) to make sure I do it right, but thought that I should drop a mail here for anyone else who isn't already aware.
Some useful links:
https://support.google.com/a/answer/81126
https://senders.yahooinc.com/best-practices/
https://www.linuxbabe.com/mail-server/setting-up-dkim-and-spf (Postfix and DNS)
https://www.linuxbabe.com/mail-server/create-dmarc-record
https://www.exim.org/exim-html-current/doc/html/spec_html/ch-dkim_spf_srs_and_dmarc.html
Testing:
https://www.mail-tester.com/spf-dkim-check
Emailing check-auth@verifier.port25.com from the domain you are testing.
Thanks
Gavin