SmtpClient.Send() - This action cannot be completed because the other program is busy. Choose 'Switch To' to activate the busy program and correct the problem

When sending an email (var x = new Aspose.Email.Mail.SmtpClient(); x.Send(mailmessage); ) with one or more large attachments after some time i get the message "This action cannot be completed because the other program is busy. Choose 'Switch To' to activate the busy program and correct the problem".

This appears to be a Windows message implicing that 2 processes require the same resource but it is not released by one of the processes.

Please, could you try to to reproduce the problem? It occurs at our clients so we are in big need to solve this soon.

Kind regards, and thanks in advance.

Jan.