The Return-Path is the address where bounce messages go. It is set during the SMTP conversation, when the sending server announces the envelope sender with the MAIL FROM command, and the receiving server writes it into the message as a Return-Path header. It is separate from the From address your readers see, and usually nobody looks at it except the systems that process bounces.
Email platforms usually set the Return-Path to an address on a domain they control, so that bounces come back to their systems and can be processed automatically: hard bounces suppressed, soft bounces counted. A message from hello@yourbrand.com might therefore have a Return-Path like bounce-123@bounces.platform.example.
The Return-Path matters for authentication because SPF checks the envelope sender’s domain, not the From domain. If the Return-Path is on the platform’s domain, SPF may pass for the platform but won’t align with your From domain for DMARC. That’s fine as long as DKIM aligns, since DMARC needs only one of the two. A custom return-path on a subdomain of yours, such as bounces.yourbrand.com, lets SPF align as well.
If you connect your own sending service or SMTP server, check where bounces go. A Return-Path pointing to a mailbox nobody reads means bounces pile up unprocessed and you keep emailing dead addresses. Either let the service report bounces back to your platform, or point the Return-Path at a mailbox that is processed automatically.
A Bengaluru SaaS company sends from news@itsbrand.com through Amazon SES. It sets up a custom MAIL FROM domain, bounce.itsbrand.com, in SES and publishes the SPF and MX records SES asks for. Now both SPF and DKIM align with its From domain.
When you connect your own sending server to MailNesto, you can set its Return-Path or link a bounce mailbox that MailNesto reads, so bounces are still processed and suppressed.