How to configure webform with smtp
Webform module uses the mail() for sending the mails. SMTP Authentication Support module allows Drupal to bypass the PHP mail() function and send email directly to an SMTP server. 

Along with SMTP Authentication Support module you would need to install the Mail system module and configure it to assign a smtp class to webform which is  step by step explained in this tutorial.

If you have SMTP module installed in your drupal site than you would have to configure the SMTP settings (/admin/config/system/smtp) for sending the email

Example (Configuring SMTP for GMAIL):

A. INSTALL OPTIONS:
 1. Turn this module on or off: (SELECT "ON")

B. SMTP SERVER SETTINGS:
 1. Server: smtp.gmail.com
 2. Backup server: (LEAVE BLANK)
 3. Port: 465
 4. Use encrypted protocol: (SELECT "Use SSL")

C. SMTP AUTHENTICATION:
 1. Username: (ENTER YOUR GMAIL EMAIL ID)
 2. Password: (ENTER YOUR ABOVE EMAIL ID PASSWORD)

D. E-MAIL OPTIONS:
 1. E-mail from address: (ENTER YOUR GMAIL EMAIL ID AS ENTERED ABOVE)
 2. E-mail from name: (ENTER SITE NAME, ITS DEFAULT VALUE IS DRUPAL VERION)
 3. Allow to send e-mails formated as Html (ENABLE CHECKBOX IF YOU WANT)

E. SEND TEST E-MAIL:
 1. E-mail address to send a test e-mail to: 
   (ENTER YOUR EMAIL ID AT WHICH YOU WANT TO RECEIVE TEST MAIL)


Instead of GMAIL if you want to use some other service provider than check their server details and the port details.