Database Mail failing with “The service queue "ExternalMailQueue" is currently disabled.” message

Some of you may experience problems while using Database Mail on SQL Server 2005 or SQL Server 2008 which will not be able to send emails to the database users. The SQL Server error logs will log following errors:   The activated proc [dbo].[sp_sysmail_activate] running on queue msdb.dbo.ExternalMailQueue output the following:  ‘The service queue “ExternalMailQueue”…