All messages from same email

Hello,

I used this guide to configure postfix

https://www.linode.com/docs/email/postf … or-ubuntu/">https://www.linode.com/docs/email/postfix/configure-postfix-to-send-mail-using-gmail-and-google-apps-on-debian-or-ubuntu/

But now all applications send email from that gmail account.

How to avoid that?