Attachments are not released until the Microsoft Dynamics NAV client is closed when you use the SMTP mail codeunit (400)

As reported in multiple blogs and user forums, the SMTP component had an issue about locking the files that was attached to sent emails. This usually always caused issues, as you would clean up your temporarily generated files after having sent them with the email attachment. According to the released hotfix KB2280492, this was occurred [...]
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