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By deliverability issues, I believe you mean outbound mails? I have the same concern but I don’t think that’s related to the cloudflare though. Its only about using gmail SMTP server to send custom domain emails without correct signing

Yes I mean outbound email being marked as spam or otherwise not being delivered to my recipient’s email.

I haven't used it much, but personally I replaced the Gmail Outbound server for Sendgrid. Inbound I still use the CloudFare system.

Cloudflare will have nothing to do with you deliverability because that is only being used for receiving emails

I do this with MailGun across a multitude of professional / company domains. Never has a problem with deliverability. MailGun has a very nice option where you can use RegEx in your routing rules, even across domains (and subjects, recipient, etc)

I think I tried exactly this configuration a few months ago. I also had deliverability issues.