Trickbot is a banking trojan that steals online banking information and personal data, ready for criminals to then commit identity fraud. As the National Cyber Security Centre puts it: Trickbot targets victims with well-crafted phishing emails, designed to appear as though sent from trusted commercial or government brands. These emails will often contain an attachment (or link to an attachment) which victims are instructed to open, leading to their machine being exploited.
Source code of one of the suspicious emails posted to Github by PHP dev Richard Bairwell revealed the full message headers, which appear to point to email firm Campaign Monitor as the source of the message.