The following TXT record:
v=DMARC1; p=reject; pct=100; adkim=s; aspf=s
at least theoretically, should harden SPF and, if present, DKIM. “=s” means “strict”. According to DMARC documentation, DMARC can be used without DKIM, and experiences with a first setup of the above without DKIM are playing out well so far.
Some info is here:
https://www.dmarcanalyzer.com/how-to-create-a-dmarc-record/
Here is an excellent tool to check if your IP or domain is on a spam blacklist (a.k.a. RBL):
http://mxtoolbox.com/SuperTool.aspx
Type “blacklist:yourdomain.com” for domain checks.
Another is here, IP only:
http://whatismyipaddress.com/blacklist-check
Both check rather large collections of blacklists, as you watch.
