You should set SMTP Server settings in Newsletters module in the administration section in Sitefinity 3.6 and Sitefinity 3.7.
Please, do the following steps:
1. Log in to the Administration and go to section Modules>Newslettes.
2. Click on the SMTP Server Settings link on the left side pane.
3. In the page which opens, enter your SMTP Server settings.
4. Click on "Test and save" current setting button.
Using the SMTP server settings and email provided in step 3, a verification email will be sent.
5. Click on the Activate button. The settings will be applied to the Newsletter Module.
If you have any problem, do not hesitate to contact us.
the Telerik team