Google Inc.'s free e-mail service will shed the final remnants of its invitation-only restrictions Wednesday, extending the reach of an increasingly popular product that has emerged as a vital cog in the online search leader's expansion efforts.

Invitations will no longer be required to join the nearly 3-year-old "Gmail" service in the United States, Canada, Mexico and a swath of Asian and South American countries where the Mountain View-based company previously limited the number of users.

With those restrictions now lifted, Gmail will be open to all comers worldwide for the first time since Google unveiled the service on April Fool's Day in 2004.

The decision to lift all invitation requirements on Gmail signals Google finally believes it has adequate computing capacity to accommodate the generous amount of free storage provided by the e-mail service after investing heavily in additional data centers. Gmail offers each account at least 2.8 gigabytes of storage — enough to fill about 1.4 million pages.