The steps to change the session timeout for SmarterMail are as follow.

Modifying Configuration File:
One need to edit configuration file which can locate any below mentioned path:
C:\Program Files\SmarterTools\SmarterMail\MRS\web.config

Open web.config file and find the following code within the <system.web> section and change the timeout value to the desired one in minutes then save the file.

<sessionState mode="InProc" stateConnectionString="tcpip="
sqlConnectionString="data source=;userid=sa;password="
cookieless="false" timeout="20" />

Changing the session timeout limit from IIS:
After changing the session timeout limit from web.config file we have to modify the session timeout from the IIS Manager for SmarterMail site too. For that perform below mentioned steps:

Open IIS manager -> Websites.
Right click on the SmarterMail website -> choose Properties.
Click on the Home Directory tab.
Click on Configuration button in the Application Settings.
Click the Options tab and edit the Session timeout so that it matches the value set in the web.config file.
Save the changes, click Apply and then click OK. Thanks,