When "metamta.can-send-as-user" option is turned off. The sender name will be set to the user name (for example, "Eric Liu"), and the sender email will be set to "metamta.default-address" (for example, "email@example.com"). This might cause some confusion on the recipient side: both emails from the users and phabricator might have the same name. For example, I have two "Eric Liu" in my inbox: one is for my own email address, and another one is phabricator's email address.
I think it would make recipient's life easier by appending a string like "via Phabricator" to the sender name when the email is actually sent from phabricator's email address.