I submitted a bug report once and it was fixed the next day, I was invited to try it out on their beta version of the website and it was rolled out on all the servers a couple days later. These guys are truly providing a fantastic service.
Similar experience. Fix with inability to respond to some crazy github-generated email addresses rolled out publicly in less than 48 hours. It helps that I was able to tell them "I am pretty sure it's your regex on valid email addresses because I can reproduce it with other emails like $this"
I also recall they fixed an issue where "--" at the top of an email I was replying to was making the whole thing look like a signature and when I tried to send it was warning me about an empty message.
That's why I'm a Fastmail customer.