I'm using gmail as a mail relay for my server (I do not have a static IP) and was wondering if it's possible to hide the mail relay email address. I'm running ubuntu as a server and using postfix for SMTP.
for example, when I send an email from firstname.lastname@example.org, I want the from address to show email@example.com, not firstname.lastname@example.org. Is this possible to do?
You should be able to send mail from a website as whatever you want, if its relayed on its way out that shouldn't change any headers that have been set.
I'm fairly sure gmail will alter outgoing headers if the sender domain does not match your google apps domain, or is different to a google email domain. If they didn't, they would attract spammers looking for free smtp with good deliverability.
You may as well register your domain for google apps, update the mx records to theirs then use their service for both incoming and outgoing. You can set it up as imap or pop for retrieval.