GMail Sender header problem and using other domains

I've been using GMail for a while now as an SMTP server to send my e-mails from other domains, no matter where I am going. What I realized later is that most of the e-mails I send are getting seen as sent by my account @gmail.com, while I'm actually trying to send them as if they were sent from @dokeos.com. Well, apparently the whole thing is quite compromised.