Emotet botnet malware was the most prevalent email-based threat in the first three months of 2019. 61% of malicious payloads delivered via email were botnets, all detected as emotet. Other threats spread over malicious emails were banking trojans, credential stealers, downloaders, and remote access trojan RATs. Email is the main method for delivering malware, which can come either as an attachment to the message or via download link included in the email body.
Source: https://www.bleepingcomputer.com/news/security/emotet-botnet-behind-most-email-based-threats-in-q1-2019/