Steps to Reproduce:
Step 1: Create a test list and click on Schedule Test List  for execution.

Step 2: Configure the SMTP settings.

Step 3: On step 3(Select Notification Settings), upon selecting the option- Send test results by email, the email customization area to enter email addresses gets enabled.

Step 4: Enter the distribution list here.

Expected Result: All the email recipients in the distribution list must receive the email notification, upon the execution of the test list.

Actual Result: Sometimes, only the first recipient from the distribution list receives the email notification, else there is no email being sent at all. 
When multiple distribution lists are entered, same results are observed.