You can’t realistically host an email server on your phone, which is what is meant. For that to work, your phone would need to be able to be woken up (to handle receiving a message) by arbitrary parties on the internet at any time, which is generally considered undesirable, quite apart from the bother caused by your phone ever being offline (sender’s server will keep retrying every so often for a couple of days, sender will generally be notified of this each time—and note the problem of the sender’s phone-hosted mail server being offline too if that’s what you were doing).