How can I send emails on behalf of my other domains without being flagged as spam?

Context

I have some domains let's call them:

• foo.com
• bar.io
• baz.net

I have email hosting for foo.com with MX, SPF, DKIM, etc set up and mail-tester deducts score due to my test email's subject and message body.

I have email forwarding for bar.io and baz.net so they both forward to foo.com.

The Question

What I would like to do is send from foo.com on behalf of bar.io or baz.net without being swallowed by spam filters. Is this possible?

I've seen various posts on the web mention setting up SPF records, and yet DMARCian seems to suggest this is typically erroneous. If I go the SPF route and emails bounce, will they forward to foo.com or will the void consume them?

I am able to edit the DNS records for all domains. If that helps.

What I would like to do is send from foo.com on behalf of bar.io or baz.net without being swallowed by spam filters. Is this possible?

Since you're referring to DMARC, you require at least authorization by SPF (assuming that's used in DMARC - DMARC requires anchoring by SPF or DKIM). Creating an SPF entry in each domain and include: the foo.com MTA (or the whole domain). Alternatively, you can redirect: each secondary domain to the foo.com entry, but that might give you less flexibility for those domains.

What I would like to do is send from foo.com on behalf of bar.io or baz.net without being swallowed by spam filters. Is this possible?

Bouncing an email is generally done by the MAIL FROM address from the SMTP session ("envelope from"). Some servers incorrectly use the header From:. The bounce message is sent to that address's MX. Bouncing doesn't use the SPF record.

Note that messages can also be rejected during the foo.com MTA's transfer attempt, so that MTA needs to be set up to properly bounce the message.