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.
Trojan.SpamBot is a detection for Trojans that are capable of sending out spam from affected systems. Spam sent out may or may not contain malicious file attachments.
This Trojan arrives on system via spam or malicious mail attachments.
Malwarebytes can remove Trojan.SpamBot without further user interaction.