Reading into Gmail sending limits for Google apps is not straight forward. Reading between the lines magic number you need pay most attention to is 500.
500 is the maximum number of external recipients you are allowed to send a single message to.
Even if you send out multiple messages to break things up you can only send to 2,000 unique recipients per day.
To keep their systems healthy and your accounts safe, all Google Apps accounts limit the amount of mail a user can send. The limits restrict the number of messages sent per day and number of recipients per message. After reaching one of these limits, a user cannot send new messages but can still receive incoming email.
If you need to send an unusually large amount of mail, such as for a marketing campaign, Google recommends you consider an alternative to regular email – something like Flashissue for example because you can then eliminate the issue of Gmail sending limits altogether.
Here’s the dirty details:
The following peak limits apply for Google Apps for Business or Education editions.
The value of these limits may change without notice in order to protect Google’s infrastructure. Limits per day are applied over a rolling 24-hour period (rather than a set time of day).
Sending limits for Google Apps for Business trial users and new paid customers may be lower than those in the table above. If your domain has fewer than six users, it may take several billing cycles to reach peak limits.
Locked out of your account? Google Apps administrators can check the User list in the Google Apps Control Panel to find details on which limits you hit and when access will be restored. In some cases, a Google Apps for Business, Government or Education account administrator can reset the Gmail suspension.
Read about sending limits for free Gmail accounts.