I am using MailGun, and the SMTP server that my MailGun account was using was being blocked by various email providers including Yahoo Mail and AOL and Apple. It seems that the email server I was using was shared and had sent too much spam to those providers. In response, I could see in the logs that the providers sent messages detailing why they were blocking my email
So I am wondering if a similar situation could be happening here, but with your private email server, and with Gmail. Maybe Gmail does not like your server for some reason? You have verified that other email providers are not blocking it?