Fastmail's security is top notch. They're using Cure53's DOMPurify and their team understand these vectors really well, with strict filtering. Their pixel tracking blocking was better than Hey's when last I checked, and I reported three leaks to Hey that were not fixed for some time. Fastmail also run one of the best bounties I have ever participated in. I would even say significantly better than ProtonMail, having participated in both bug bounties.
Nice to hear that, I'm a Fastmail customer and the Android up is subpar on offline access (or unreliable connections, looks like it's basically hitting the network for everything. Knowing that at least security-wise it's a good product makes me happier.
Some more integration with other FastMail services (like calendar or "disk space" for files) mainly. Also normal MUAs on Android are usually ugly as hell, I still have to find one that I really like. But the online-only bug of FM client is really becoming an issue worth switching.
shrug dunno what other mail clients have but I've used Fastmail's email client for over a decade and never thought "wonder if someone else's is better".
