Utility.SendEmail (CSOM - C#) 'To' field is blank for few users

Hello,

We're facing weird issues with Utility.SendEmail (CSOM - C#) for few users. In my EmailProperties object, I have 'To' & 'Cc' properties

I have cc'ed myself to check if email is sending properly. I can see that for few users 'To' field is getting blank in outlook and they are not receiving emails. 

I am using web.EnsureUser() function before adding user to EmailProperties object. For the list of users having issues, web.EnsureUser() is working fine as well

Please help!