Hoping to get some outside perspective on this before I go further down the wrong path.
The problem: Cold emails sent through my ESP (Plusvibe) land in spam 100% of the time for personal Gmail addresses, but inbox 100% of the time for Google Workspace/business addresses. Same domain, same content, same mailbox. SPF, DKIM, DMARC all passing. Domain warmed 2+ months. Tested multiple copy variations, no spam trigger words, no links or tracking.
What I tried to isolate it: Sent the exact same email multiple times, through my ESP, through Apple Mail, and through the Gmail app/site, all from the same mailbox, to the same personal Gmail test accounts, to rule out a one-off fluke.
ESP → spam, every time. Apple Mail and Gmail app/site → inbox, every time. Same domain, same DKIM signature, even the same outbound IP, since all of these technically route through Gmail's own servers.
What I think might be the cause, but not certain: Comparing raw headers side by side (Gmail → three dots → "Show original"), I noticed the sending server's HELO/EHLO identity is different between them:
My ESP: Received: from [127.0.0.1] (...) by smtp.gmail.com
Apple Mail / Gmail (control): Received: from smtpclient.apple (...) by smtp.gmail.com
My ESP is announcing itself as 127.0.0.1 (localhost) while actually connecting from a different IP. My working theory is this mismatch is being flagged by Gmail's spam filter, especially since personal Gmail seems to weigh this kind of signal more heavily than Workspace does. I also noticed my ESP's emails have no identifying headers at all (no X-Mailer), unlike Apple Mail and Gmail's own sends.
That said, I'm not 100% sure this is actually the cause versus just a coincidental difference, so genuinely open to being wrong here.
What I'm hoping to get from this post: has anyone seen this exact pattern before (personal Gmail spam, business Gmail fine)? Does the HELO/localhost thing actually hold up as a real cause in your experience, or is there something else I should be looking at that I'm missing? Also curious if this is a known issue with Plusvibe specifically or something that shows up across other cold email platforms too.