ActionMailer with Google Apps

I am trying to send email from a Google Apps account. Without specifying
an settings whatsoever, when attempting to send email, my postfix server
will simply relay the request to google, which I assume is much slower
than connecting to Google's smtp server directly. I eventually decided
to try this "direct connect" approach because I was getting an error
when trying to send emails to an address@my_domain.com. Specifically,
postfix spit out this error message: "Recipient address rejected: User
unknown in local recipient table". Now, I could just change some postfix
configuration settings, but I decided to try the "direct connect"
approach first.

Here are my smtp settings in environment.rb:

ActionMailer::Base.smtp_settings = {
    :tls => true,
    :address => "smtp.gmail.com",
    :port => "587",
    :domain => "forwardfab.com",
    :authentication => :plain,
    :user_name => "USERNAME",
    :password => "PASSWORD"
}

I have a few issues with this approach, and I have a few questions
concerning differences between having a local postfix server relay the
request to google, as oppposed to connecting to the Google smtp server
directly.

First, does connecting directly to the smtp server somehow validate that
the sender of the email is who it says it is, in terms of spam filters?
Do I lose legitimacy (in terms of spam filters) by having my local
postfix server relay the request?

Second, the username i specify in the smtp settings is the user that
appears in the "from" header in any emails sent. This is not very ideal,
because emails need to come from multiple accounts throughout the
application. Is there a way to do this, short of toying with settings in
my google mail account? Using the "relay approach", I can send from
pretty much whoever I want.

Third, I'm not crazy about having username/password info in config
files.

Thanks a million in advance to anyone who tackles this monstrous post.