Custom URL not working for me

I am a paid user with Zoho and trying to use mail with custom url, however it is not working for me.

The custom url I want to use is mail.domain.com. I have added both, my main domain as well as mail.domain.com in zoho control panel. Both are verified and mail hosting enabled on it.

Added CNAME record at my DNS provider for mail.domain.com pointing to business.zoho.com.

Now when i visit http://mail.domain.com it works fine. However, once I log in... the url turns out to be https://mail.zoho.com/biz/index.do?Requested_Domain=mail.domain.com

I have disabled 'Always use secure connections' from the preferences. However, still not working.

Would appreciate your assistance in this matter.