Mail::send() doesn't close smtp connection

Hello!
I use Mail::send() with smtp backend to send mails from php script on my FreeBSD server.
It works fine when everyting is ok.
But if there's an error (E.g. SMTP authentication failure) Mail::send() finishes with error and leaves tcp connection with smtp server open.
Is this a feature or bug?
My script tried to send thousands of mails and thus hit the system limit of open files (network connection is concidered as an open file). As a result other services (such as database) became unavailable too.
So... Is it the right behaviour for Mail::send() not to close the connection?
Thanks.