I want to use zimbra as internal and external mail, for example

user juan has a internal mail account (juan@midominio.local) and also has an external mail (juan@miISP.com)

my question is, can configure Zimbra to use two accounts? and how is it done?

so it wants Zimbra automatically identify which of the two accounts should send the respective post.

I hope you understand me, my English is not very good