Phishing emails typically masquerade themselves as reputable identities to trick people into providing sensitive information and credentials. Despite advancements in cybersecurity, attackers continuously adapt, posing ongoing threats to individuals and organisations. While email users are the last line of defence, they are not always well-prepared to detect phishing emails. This study examines how workload affects susceptibility to phishing, using eye-tracking technology to observe participants' reading patterns and interactions with tailored phishing emails. Incorporating both quantitative and qualitative analysis, we investigate users' attention to two phishing indicators, email sender and hyperlink URLs, and their reasons for assessing the trustworthiness of emails and falling for phishing emails. Our results provide concrete evidence that attention to the email sender can reduce phishing susceptibility. While we found no evidence that attention to the actual URL in the browser influences phishing detection, attention to the text masking links can increase phishing susceptibility. We also highlight how email relevance, familiarity, and visual presentation impact first impressions of email trustworthiness and phishing susceptibility.