Hello @Sally1202 ,
Test email deliverability can have a lot of variables, however the main issue that usually occurs is that the receiving server doesn't trust that we (a 3rd party server) are sending you an email from your own email address, outside of your server. It's generally a good idea to set up additional authentication for your emails, including for tests, whether that's through us if you have a free domain (e.g. Gmail, Yahoo, AOL, etc.) or via self-authentication if you have your own domain (such as through a business or other organization). Using our authentication for a private domain can cause issues depending on the security settings of the receiving side, which is why we recommend using self-auth if you have your own.
Safelisting can also aid in your test delivery, especially safelisting our domains if you have a company or government network that's managed by an IT team.
If you're wanting more in-depth, specialized insight on your current deliverability, it'd be worth speaking with one of our Deliverability agents.