This is exactly the same question that my customer had when they upgraded to CRM 2013. In CRM 2011, they could go to Personal Options and in the E-Mail tab could see the following screen
If they check the option “Allow E-mail router to use my credentials to send and receive e-mail on my behalf”, they would get an option to enter the username and password. However once they upgraded to CRM 2013, they could not find that option at all in the e-mail tab.
For users who have faced this, don’t be tensed. This option is still available. However in a round-about way.
In CRM 2013 you would need to go to your user record. Once you open the user record, you would find that a default mailbox has been created with your name. For e.g – Check the below screenshot for my user record. You would typically find this under Email Configuration section.
Click to open the mailbox. Once you open the mailbox, you could see the section where you can enter your credentials.
Hope this helps!