Word on the street is, if you're running an MTA that utilizes pipelining aggressively, you're probably having trouble sending email to Gmail right now.
Here's an example of the error message received when messages are being deferred: (host aspmx.l.google.com[18.104.22.168] said: 451 4.5.0 SMTP protocol violation, see RFC 2821 (in reply to end of DATA command))
The issue is clearly affecting people running Postfix. I'm unclear on whether or not it is affecting others.
More on the issue, including a fix for Postfix users, can be found over on the Word to the Wise blog.