ASP.NET Core - SendMail from Gmail account appears in send items but not in receiver's inbox

When I send emails trough our ASP.NET Core app, we are successfully sending emails trough an Office365 account to all kinds of email providers (Gmail, Hotmail,...) but when we are sending mails trough a Gmail account, only the @gmail.com accounts recieve the email (so, among others, not the @hotmail.com emails). We can see the e-mails in our send items, but they are not in the recievers' inbox.

What could possibly cause this? Could this be related to a CNAME / DNS issue?

Extra info: We host our app with Google Cloud Services