SPF domain
The domain whose SPF record is checked: the domain of the SMTP MAIL FROM address, or the HELO name when that address is empty.
What it means
SPF authenticates the envelope sender given in the SMTP MAIL FROM command, the address bounces go to and that later appears in the Return-Path header. RFC 7208 calls it the MAIL FROM identity. When the envelope sender is empty, as for bounce messages, SPF checks postmaster at the HELO name instead.
DMARC uses only this identity from SPF and calls the validated domain the SPF domain. To count toward DMARC, it must align with the From domain: share its organizational domain under relaxed alignment, or match it exactly under strict.
Sending platforms often use their own bounce domain in MAIL FROM, so SPF passes but does not align. Configuring a custom return-path (bounce) subdomain under your own domain fixes that.
Related terms
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Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes