Multiple virtual domains but outbound mail shows only 1 domain

[jeffs...@gmail.com]

Hello, I'm sure this has been asked many times before, I know that because it's a common sense question with very little coverage in the net.

Pretty much all tutorials explain how to setup virtual domains to *receive* for various domains, but there is a dearth of information on the mechanics how to make *outbound* email contain the domain name of the sending virtual domain.

I mean, this seems like a pretty straightforward expectation. . . no? Postfix receives and parses for individual domain users but when they send outbound, the machine's domain name is appended.

/etc/mailname: example1.com

jo...@example1.com
su...@example2.com
har...@example3.com

jo...@example1.com easily receives email destined for him as does all other virtual users according to their domain whether it be example1, example2 or example3.

Yet when susie or harris send outbound, the recipient sees:

su...@example1.com and har...@example1.com yet they are not part of those domains.

Isn't there a simple method for correcting this?

What must I do in my setup to correct this?

Postfix

postconf -d | grep mail_version
mail_version = 2.11.0
milter_macro_v = $mail_name $mail_version

Ubuntu 14.04