Trojan.Agent refers to a generic Trojan malware detection. It is used for detections that are either associated with no specific malware families or not enough information is available to pinpoint the malware family.
It is packed with UPX2
Malwarebytes can remove Backdoor.SpamTool without further user interaction. It is recommended to do a full system scan since this backdoor may have been used to install more malware onto the system and was maybe installed by other malware.