Can I use 'mail.domain' in my mail client?

Yes, absolutely!

For compatibility reasons and to help ease migrations, we automatically setup mail.domain for each of your packages. You'll just need to make sure to add the free SSL to the hosting package so that mail is sent over an SSL/TLS connection. 

Any clients already using these hostnames can continue to do so when the account is moved to 20i, and any new accounts added at 20i will have the same functionality.