SINGLE_EMAIL_LIMIT_EXCEEDED. What does it mean?

Per the Governor Limits, you can only send 5000 single emails a day (or less, depending on license).

Using the API or Apex, you can send single emails to a maximum of 5,000 external email addresses per day based on Greenwich Mean Time (GMT). Single emails sent using the Salesforce application don't count toward this limit. There’s no limit on sending individual emails to contacts, leads, person accounts, and users in your organization directly from account, contact, lead, opportunity, case, campaign, or custom object pages.

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_gov_limits.htm

Here is my attempt to combine all of the answers given, and to interpret the SF Documentation provided on email limits.

A SINGLE_EMAIL_LIMIT_EXCEEDED exception is thrown when the daily Messaging.SingleEmailMessage limit is exceeded. This exception can be thrown in other cases as well, like when email deliverability is not set correctly.

Each SingleEmailMessage sent counts toward the limit, even if it is sent to the same email address.

A Dev Edition org has a single email limit of 15 messages. (assuming 1 recipient per email)

The single email limits don't take unique addresses into account. For example, if you have [email protected] in your email 10 times, that counts as 10 against the limit.

In Developer Edition organizations and organizations evaluating Salesforce during a trial period, your organization can send single emails to a maximum of 15 email addresses per day.

A non Dev org has a single email limit of 1000 messages. (assuming 1 recipient per email)

Using the API or Apex, you can send single emails to a maximum of 1,000 external email addresses per day based on Greenwich Mean Time (GMT).

I had this error a couple weeks ago. For me, it has something to do with the Sandbox not being configured to deliver email.

My resolution was to go into Admin Setup > Email Administration > Deliverability

There is a setting called 'Access to Send Email', I moved that to 'All email' and it worked. https://help.salesforce.com/apex/HTViewHelpDoc?id=emailadmin_deliverability.htm&language=nl