auth := smtp.PlainAuth("", user, pass, server)
This is using the authentication method PLAIN. Unfortunately smtp-mail.outlook.com does not support this authentication method:
> EHLO example.com < 250-AM0PR10CA0007.outlook.office365.com Hello ... < 250- ... < 250-AUTH LOGIN XOAUTH2
Thus, only LOGIN and XOAUTH2 are supported as authentication methods.
Python supports LOGIN so it will succeed.
Golang smtp does not support LOGIN. But this gist seems to provide a working fix by adding this missing authentication method.