Configure SPF/DKIM to Resolve Gmail Error 550-5.7.26 Unauthenticated email

Gmail keeps bouncing my messages with the dreaded “550-5.7.26 This mail is unauthenticated” notice. I send through a Postfix server and manage all DNS zones inside DigitalOcean’s control panel.

I already know the problem: my domain lacks working SPF and DKIM records. I need an expert who can generate the correct tokens, publish them in DigitalOcean DNS, and tie everything back into Postfix so Gmail finally recognises the messages as authenticated.

Scope of work
• Create or repair one SPF record that aligns with my current sending path.
• Generate a DKIM key pair, add the TXT record in DigitalOcean, and wire it up to Postfix (OpenDKIM or equivalent).
• Verify both records with common testing tools (Google Postmaster, dig, etc.) and confirm Gmail accepts the message without the 550-5.7.26 error.

Please keep changes minimal and document anything you touch so I can maintain it later.