I have a VPS server, let’s say domain.com, that is configured to forward all email messages to my gmail address. For example person@yahoo.com sends an email to contact@domain.com, that is forwarded to my email address, me@gmail.com.
Well, it’s up to the email client to set the email’s from address.
So you’d need to change your email client to use a different address. If you’re sending those messages from the Gmail web interface – they offer a way there to set the address emails are sent from.