In addition to what Jonathan is recommending you should also check your anti-virus setting (if you have AV on your SQL Server). By default McAfee blocks Database mail through it's access protection setting, prevent mass mailing worms from sending mail. When this happens you get the error you listed. You need to add database mail (DatabaseMail90.exe) as an exception to the rule.
Jack Corbett
Consultant - Straight Path Solutions
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