The code sandwiched between the opening and closing tags here will only affect Microsoft Outlook 9 or later clients. Note that because it says mso rather than IE, it does not affect Internet Explorer.
A quick breakdown of Outlook versions:
- Outlook 2000 – Version 9
- Outlook 2002 – Version 10
- Outlook 2003 – Version 11
- Outlook 2007 – Version 12
- Outlook 2010 – Version 14
- Outlook 2013 – Version 15
For a more detailed explanation, you can see this helpful page from Mailchimp
Source: javascript – Purpose of using conditional comment