Reference
Email authentication glossary
Plain-English answers to every DMARC, SPF, DKIM and MTA-STS parameter — what each string means, valid values, and how to fix it.
DMARC
p=noneWhat does p=none mean in a DMARC record?p=quarantineWhat does p=quarantine mean in a DMARC record?p=rejectWhat does p=reject mean in a DMARC record?v=DMARC1What does v=DMARC1 mean in a DMARC record?pctWhat does the pct tag do in a DMARC record?spWhat does the sp tag mean? DMARC subdomain policy explainednpWhat does the np tag mean in a DMARC record?adkim / aspfWhat do adkim and aspf mean? DMARC alignment explainedfoWhat does the fo tag mean in DMARC? (fo=1 explained)riWhat does the ri tag mean in a DMARC record?rfWhat does the rf tag mean in a DMARC record?dmarc=passWhat does dmarc=pass mean in an email header?dmarc=failWhat does dmarc=fail mean and how do you fix it?SPF
v=spf1What does v=spf1 mean at the start of an SPF record?-all vs ~allSPF -all vs ~all: hardfail vs softfail explainedinclude:What is the SPF include mechanism and how does it work?aWhat does the a mechanism do in an SPF record?mxWhat does the mx mechanism do in an SPF record?ip4:What does ip4 mean in an SPF record?ip6:What does ip6 mean in an SPF record?ptrWhat does the ptr mechanism do in an SPF record (and why is it deprecated)?exists:What does the exists mechanism do in an SPF record?redirect=What does redirect= do in an SPF record?exp=What does the exp modifier do in an SPF record?permerrorWhat does SPF permerror mean and how do you fix it?temperrorWhat does SPF temperror mean?10-DNS-lookup limitWhat is the SPF 10-DNS-lookup limit and how do you fix 'too many DNS lookups'?DKIM
selectorWhat is a DKIM selector and how do you find it?v=DKIM1What does v=DKIM1 mean in a DKIM record?d=What does the d= tag mean in a DKIM signature?p=What is the p= tag in a DKIM record?a=rsa-sha256What does a=rsa-sha256 mean in a DKIM signature?c=relaxed/relaxedWhat does c=relaxed/relaxed mean in a DKIM signature?t=yWhat does t=y mean in a DKIM record?h=What does the h= tag list in a DKIM signature?b= / bh=What are the b= and bh= tags in a DKIM signature?dkim=failWhat does dkim=fail mean and how do you fix it?MTA-STS
v=STSv1What does v=STSv1 mean in an MTA-STS record?mode:What does mode mean in an MTA-STS policy? (enforce vs testing vs none)max_ageWhat does max_age mean in an MTA-STS policy?mta-sts.txtWhat is the mta-sts.txt policy file and where does it live?v=TLSRPTv1What does v=TLSRPTv1 mean in a TLS-RPT record?