On your own email server make sure that you have correctly setup dkim, spf, reverse dns, and the ip of server is not on any known denylist - but it still doesn’t guarante that your email server would not get flagged as a spam source.

Or maybe setup transport rules that would try to deliver most emails directly, but to major providers like Gmail/outlook which are quite picky via smtp relay - at work we are using AWS SES to do that, it is not that expensive, but it depends on the volume.