Switching to SMTP not working

[Aramaech]

I'm currently using PHP for my registration email sending method, which has proved extremely unreliable.  
Of about 150 registrations we got one day, only 100 or so registration emails actually sent.
I'm trying to switch to SMTP but I get an error message when I do saying a test message could not be sent.

Does anyone know how to solve this?
 

[bern5]

check the SMTP settings from your host 

[Jim M]

Double check your host in not blocking the port needed to connect and that your connection details are correct.