message not sent error

On every email I send out from EM Client I get an error message that the email was classified as Spam and cannot be sent.  This happens on emails that I originate as well as forwarded messages.  However, I know my emails are sent and that they are being received by the intended recipient as I’ve checked. How can I stop this?  It is really annoying and makes me feel the program is unreliable.  If I use Webmail I don’t get this type of message.