550-5.7.1 "unsolicited mail"

Until a few weeks ago, I was able to send email mostly anywhere using my local postfix.  To me that's the most convenient configuration because I frequently switch networks and I don't like to change my SMTP server every time.  So I configured postfix on my laptop and I'm using "localhost" as my SMTP server.

But suddenly, a few popular email servers such as GMail or Yahoo now seem to reject my email with this message:

<foobar@gmail.com>: host gmail-smtp-in.l.google.com[66.249.93.27] said:
    550-5.7.1 [11.22.33.44] Our system has detected an unusual amount of
    unsolicited 550-5.7.1 mail originating from your IP address. To protect our
    550-5.7.1 users from spam, mail sent from your IP address has been
    550-5.7.1 rejected. Please visit 550-5.7.1
    http://www.google.com/mail/help/bulk_mail.html to review 550 5.7.1 our Bulk
    Email Senders Guidelines. u1si8404313uge (in reply to end of DATA command)

Err, what should I do here?  I'm pretty sure that no SPAM has been sent from my IP.  What's "unsolicited mail" anyway?  Do I need to ask one to solicit an email from me so I am able to write him? :-)  This stuff is quite annoying...  Like any security restrictions, SPAM filters affect legitimate use as well.

I'm now forced to login to GMail when I want to send a message to someone at GMail, which kind of sucks..