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.
Add a CNAME record
Login to your domain provider and add this CNAME record in the DNS editor.
Host / Name 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.