is it possible to mask zoho url with my own domain?

is it possible to mask zoho url with my own domain?

Hi, I've just signed up to trial zoho, so im comletely new to it.
I have my own domain & have pointed it to zoho & can send & receive emails fine.
When I go via a proxy that restricts certain types of URLs (in particular email) I get denied access.
I can get to the logon page which is at subdomain.mydomain.com, but when I enter credentials, it logs me in, but redirects to mail.zoho.com/biz/etc
Is there any way to 'mask' the mail.zoho.com URL with my own URL so that I can get access via the proxy?
Changing the proxy to allow me access is not an option unfortunately.
I'm on the  Mail Free plan if that makes any difference.

Any help appreciated.
Thanks,
Dave.