86.SOLUTION : This error, which is usually random, means that your SMTP server is checking whether the recipient name actually exists before sending your mail. If teh recipient's mail server is down or unavailable for some reason, your SMTP server cannot verify the address and may reject the relay. If this is the case, the message will not be sent when the receiving server is again available. You actually cannot do anything about the error as this is usually a problem with ISP.