Custom MAIL FROM lets you control the hidden "return path" address that email servers use for bounce handling. Setting it to a subdomain of your own domain improves SPF alignment and is a signal of a mature, well-configured sending setup.
Every email has two "from" addresses, and they're different things:
Most recipients never see the MAIL FROM address. But inbox providers check it during SPF authentication, and it affects whether your email passes alignment checks.
SPF checks the MAIL FROM domain, not the From: header domain. By default, when PristineSend sends on your behalf, the MAIL FROM is on a shared PristineSend subdomain. That means:
DMARC requires either SPF or DKIM to pass with alignment (matching your From: domain). Since DKIM alignment is already handled by your DKIM setup, SPF misalignment alone doesn't break DMARC — but having both aligned is the gold standard, and removes the "sent via" annotation that some inbox providers show.
Custom MAIL FROM uses a subdomain of your domain — typically bounce.yourdomain.com or mail.yourdomain.com. You'll need two records for it.
The MX record tells other mail servers where to send bounces for your custom MAIL FROM subdomain. Without it, bounces can't be delivered and some providers will treat your domain as misconfigured.
The TXT record authorises PristineSend to send mail from your MAIL FROM subdomain. This is what makes SPF pass for your domain instead of PristineSend's domain.
Important: if your root domain (yourdomain.com) already has an SPF TXT record, this subdomain record is completely separate — it won't conflict.
The exact subdomain name, MX target, and SPF include value are shown in your PristineSend settings once you request a custom MAIL FROM domain.