I bet we are, if we're talking about Gmail we're definitely off topic within this category. Here's more info about Google's Sandbox Effect http://en.wikipedia.org/wiki/Sandbox_Effect .
I think its the google sandbox theory - hence in the SEO section. The google sandbox "theory" is basically the period of time in between when you get indexed . This can take for ever or maybe 24 hours. Some people have created backlinks for ages and still not been indexed for while others with 1 backlink of no significance can get indexed much sooner. In other words its like the "limbo" phase before being indexed which you cannot control, how ever much you do. That dosent mean you should do nothing, be patient, keep building links and it will happen. Hope that helps.
If you are talking about Google sand Box then read from here http://forums.digitalpoint.com/showthread.php?t=71522 here is complete information about google sandbox
Sandbox is kinda filter applied by Google to new websites so that new websites can not perform better on popular words than older ones. By sandbox Google is prioritizing old websites.
I really though this person is asking for the sentbox/outbox of his/her gmail account. So he/she really means about Sandbox. How do Sandbox differs to other services?
So, as I can understand, Google sandbox is not very good thing for website owners. So what we should do to avoid our website get into Google sandbox?
A thing I noticed it helps is making a sitemap and submitting it to the main engines. Some of the website's I work for have gone from N/A to PR0 in two days more or less.