Deliverability option in Email Administration

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After Spring ’13 when we create any new sandbox then salesforce set the “Deliverability” option of email administration by default to ‘System email only’.
And when we try to send email via apex it will generates a run time error “NO_MASS_MAIL_PERMISSION, Single email is not enabled for your organization or profile. Single email must be enabled for you to use this feature.”

to remove this error we need to go setup -> Email administration -> Deliverability 
then there is an “Access level” picklist ,change its value to “All email”( Allows all types of outbound email.Default for newly created non-sandbox organization)

The “System email only” setting is especially useful for controlling email sent from sandboxes so when do any development work its does not send any test mails.

for more reference : 
http://ap1.salesforce.com/help/doc/en/emailadmin_deliverability.htm