If one sends too many emails, or ccs to too many people, or gets too many bounces on the mails they send out, the user may find their Gmail account suspended for an unknown period of time. This is known as the Gmail sending limit.
There's a lot of discussion about this in the Gmail Help Discussion Group if you search for "sending limit".
What is the limit?Edit
- In an effort to fight spam and prevent abuse, Google will temporarily disable your account if you send messages to more than 500 recipients or if you send a large number of undeliverable messages. If you use a POP or IMAP client (Microsoft Outlook or Apple Mail, e.g.), you may only send a message to 100 people at a time. Your account should be re-enabled within 24 hours.
The number of bounces (undelivered messages, as explained above) also have an effect.
What is the solution?Edit
A common suggestion is to use Google Groups or create a Google+ Community if you need to communicate with a large number of people. However, for Gmail users who have just hit this limit, it's a pretty bad time to be trying to push another Google product on them.
You could try using the form to report a technical issue, but there is no evidence that this will shorten the suspension period.
How long before I can send mail again?Edit
This is unknown. Most replies in the Google Groups suggest 24 hours, though many say 24 to 48. Other users report periods of three to seven day outages.
The discussions at Google Groups highlight many issues not just with the limit, but also with Google's approach to this. There is no warning that a user is approaching a limit. Most users are not aware these limits exist. There is nothing to say when your account will be re-enabled.