SORBS does not intend for their list to block all Gmail messages. In fact, they recommend that their list should not be used as 'stand-alone signals' in blocking spam, but unfortunately, this recommendation is sometimes ignored. As a result, some messages sent from Gmail might be delayed, or even bounced back (in which case you would receive a failed delivery notification).
We take every measure to make sure that all valid messages are delivered, and we're working hard to keep delivery problems to a minimum. While we continue to address this problem, we ask that Gmail users experiencing delivery delays or Gmail users whose messages are bouncing back to their addresses, to please inform the recipient's mail provider about the issue.
If you are an email administrator, learn more about what you can do to make sure Gmail messages are delivered to your users.