Change the email subdomain

For some reason, our emails all use reallylongformalnameofmybiz.zohodesk.com as the domain. Is it possible to change it to shortname.zohodesk.com? I was able to change the portal name, but it didn't apply to these emails?

I found the following, but it doesn't explain how to change it:

"When you sign up for Zoho Desk, you are provided with a default support email address that looks like support@mycompany.zohodesk.com. The mycompany part of the email address is to be replaced with the portal name of your Zoho Desk." - https://help.zoho.com/portal/kb/articles/setting-up-your-email-channel