you need set up smtp only, when you use a smtp service, you can check here, if your domain or server ip is blacklisted: https://mxtoolbox.com/blacklists.aspx

also you can check on your server, if mail sending is activated

if you have not many people on your site, i recommend, you should use a smtp service (free plan) to test the mail delivery, if it works, the problem is on your site.

i use this: https://www.smtp2go.com/ it is very simple to set it up in una, with free plan, you get 1000 emails per month, that is enough for testing