Emails show as being sent from localhost

I have a domain which is a addon domain. In the root directory of this addon domain I have installed a wordpress. Everything is working fine with the exception of one thing I can not seem to understand.

When I set up the blog the admin email is a gmail. I get a notification email from wordpress for a user registration etc. it does goes to the gmail but shows as being sent from hostusername@host145.hostmonster.com
This also shows as the “sent from” email when a user receives an email from the wordpress as well.

I do NOT want this to show but rather all emails be shown as sent from the email I specified in the admin account when I installed the blog. I contactedmy host and they told me to install the “mail form” plugin however that has not solved my problem. they still show as being sent from my webhost email. I have searched for answers but havent turned up any real answers or even someone with this problem.