'Failure sending mail. Unable to read data from the transport connection: An existing connection was

Hi everyone I have implemented some mail sending functionality to send(Posted Sales Invoices) with custom email body, when I try to send multiple it send few of them and show a message that enables sending email If I try to debug it shows them following error.
"'Failure sending mail.
Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.'"

Please suggest a solution design change or code improvement thanks