Make sure you get 10/10 in the mail test, you have there one with -3. with red solve it. Also some providers require a warmup of the up, make sure you have valid dkim and spf from outside with validations.
And depending on to where you send the email, also the domain will be questionable the reputation, age and so on, nowadays its not a game to get a guaranteed mail in inbox on every mail provider, there are online mail testers that will allow you to validate multiple mailboxes at once and see where you mail will get.
About sending reputation google and microsoft have own tools, https://sendgrid.com/blog/5-ways-check-sending-reputation/ .
try also https://mxtoolbox.com/deliverability . and they have many more tools to help debug issues with mail server.
