When emails sent through an app end up in spam or bounce with errors such as 554 5.7.1 or 5.1.8, it is often due to reputation signals, header manipulation, or issues with the SMTP infrastructure rather than problems with the code. This guide provides steps to diagnose and address these issues, such as confirming whether the problem occurs only when sending through the app, checking if headers are being modified, understanding the dual-SMTP model used by Microsoft and Google, and considering whether link tracking is causing deliverability issues. It explains that shared SMTP pools can affect IP reputation, leading to filtering, and recommends checking the full headers and consulting diagnostic tools to identify the root cause. The guide emphasizes that most email deliverability problems are due to provider filtering decisions rather than issues with Nylas, and suggests referring to earlier posts or the Nylas Developer Docs for more in-depth guidance.