There are many details of which we need to take care when we set the DNN SMTP. Using the right settings combination in SMTP is very important.

Common SMTP setting

For most E-mail account, we need to provide SMTP Authentication information when we try to use SMTP server，so for most E-mail account, you should choose "Basic" under “SMTP Authentication:” and fill the "SMTP Username" and "SMTP Password"

For DNN 4.x

Step one:

"Admin->Site Settings -> Languages"