Use a custom domain for the webmail
If you're a company or a reseller, you may want your users to access the webmail from a different address than postale.io/webmail. Premium Plus plans offer that option.
Try with webmail.yourdomain.com.
Add a CNAME record
Login to your domain provider and add this CNAME record in the DNS editor.
Host Value webmail mail.postale.io.
That's it! Once the new record has propagated you'll be able to access the webmail from https://webmail.yourdomain.com. And yes, it works over https.
Host 'mail' instead of 'webmail' also works, in case you prefer that one.
As a Premium Plus user you can also customize the branding of both the webmail and the admin interface (white labeling).