Cross domain issue in iframe

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ASP.NET

Javascript

IFrame

, +

I own a website, say "http://a.mydomain.com". I have a webmail for my domain at "http://webmail.mydomain.com".

I want to display the webmail in an iframe inside my website. I provided a link in website which opens webmail in an iframe inside a page in my website. Both have same login credentials. I already managed to auto-login into webmail using my site's login credentials.

Problem:
When iframe is loaded in my website, there is permission issue related to cross domain. So the webmail keeps on loading... and shows a continuous progress bar.
Console error shows "SecurityError: Permission denied to access property "xxx" on cross-origin object"

But when I open webmail in a new tab on clicking link then webmail loads perfectly in new tab.

I want to display webmail inside an iframe in my website's page. How do I do that? I have control on my site "http://a.mydomain.com" but not on "http://webmail.mydomain.com".

Thanks!

What I have tried:

I have found the option to execute below line on both websites:

document.domain = 'mydomain.com'

But I do have control only on my website and not on webmail.