How do custom return-paths affect SPF?
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When you send email, there are two different "from" addresses in play. The one your recipients see in their inbox (the From header) and the one mail servers actually use behind the scenes (the Return-Path, also called the envelope sender or MAIL FROM). SPF validates against the Return-Path domain, not the visible From domain.
A custom Return-Path is when an ESP or mail platform sends email on your behalf using a subdomain or a different domain for the envelope sender. For example, your visible From might be hello@yourcompany.com, but the Return-Path might be something like bounce-123@em.yourcompany.com or bounce@mailchimp-send.com.
SPF will check the Return-Path domain's SPF record, not your main domain's record. So if you have a valid SPF record on yourcompany.com but the Return-Path is using em.yourcompany.com, you need an SPF record on em.yourcompany.com too. If that subdomain doesn't have one, SPF fails even though your main domain is perfectly set up. This is also where SPF alignment comes in. For DMARC to pass on SPF alignment, the Return-Path domain has to match your From domain (or be a subdomain of it under relaxed alignment). If your ESP is using their own domain for the Return-Path, SPF alignment fails even if SPF itself passes.
Most major ESPs handle this transparently if you set up a custom subdomain for sending. Check your current SPF record with our free SPF checker and look at what domains are actually covered.
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