Customer has already capped the drain after pouring bleach down it. The mildew smell is evident in the room but does not seem to be coming from the drain line as it is capped. I pulled the cap off and it smells of bleach. I looked inside the air handler and drain pan for any mildew but the equipment is only a few years old and in very clean condition. Customer has an air scrubber installed already. I called the service manager to help me think of other possible solutions or causes. If the customer feels very strongly the smell is coming from the drain then the drains could run into a condensate pump that then ejects the water. A whole house dehumidication system would help keep the basement dry and prevent any mildew from forming. I recommended service membership for the 3 systems. Customer to contact a water proofing company for input since it is my professional opinion that the odor is not originating from the drain.