The E-mail queue module was designed to enable the sending of large amounts of e-mails, e.g. when sending newsletter issues to subscribers, without the risk of losing any of the e-mails due to errors. If an e-mail is not delivered successfully, it remains in the queue so that it can be resent later. The e-mails in the queue are sent automatically, no manual sending is necessary as this is handled by the Send queued e-mails scheduled task.
E-mails in the queue can be monitored or manually managed through the administration interface. Please see the Developer's Guide -> Modules -> E-mail queue -> Administrating the e-mail queue topic for more information.
To utilize the e-mail queue, authorized users have the option of Sending mass e-mails to large amounts of users. Certain other modules, such as Newsletters, also make use of the e-mail queue. All other e-mails sent by the system can be configured to either use the e-mail queue or be sent directly to the SMTP server, as is described in the Developer's Guide -> Modules -> E-mail queue -> Settings topic.