I’ve been with FastMail since at least 2003. During that period they have proven to be more reliable than corporate provided Exchange/IMAP servers, iCloud, and Google. However, during that period, I believe relatively early, at least 10 years ago, they did have one serious outage, where they were down for the good part of a day. However, I did not lose any email and after that outage they completely reconfigured their data centre to build in additional redundancy. I haven’t experienced an access issue of any type since then.
My original interest in FastMail was because I was a user of the now long gone Mulberry app which had strict compliance with and robust support of IMAP standards. This included such advanced features as server side scripting with sieve.
FastMail is easy to setup with your own domains. In terms of integrating with other mail, that too is rather straight forward and I have never had an issue. I use them with iCloud, Gmail, and corporate accounts. FastMail tends to be my primary. Although, after the outage back in the day, I reconfigured my mail so that all messages were ultimately routed to both FastMail and Google. Although, I very rarely use Google. I have used it a couple of times for search.
FastMail has a better than average iOS app. Although, I tend to use both the Apple Mail app on iOS and Mac. However, on iOS I sometimes use FastMail’s, primarily for search or some IMAP administrative type things.
In terms of my original post in this thread, FastMail supports the concepts of your having an account, aliases, and personalities. Your account has to be with them but your aliases and personalities can be almost anything you would like and there is no requirement that a domain you are using be hosted with them. Essentially, as long as you can authenticate your account with their SMTP server you can send and receive for any identity you have setup with them. Including aliases or personalities which are setup on the fly.
I believe one area in which some criticised there offering was that after the introduction of the iPhone, FastMail was late in being able to support push. I was able to work around this because I also had an iCloud account. FastMail’s server side scripting allowed me to direct messages for which I wanted notifications. It also allowed me to send SMS messages to my phone if I was in a country where I didn’t have data access.
The one feature that I don’t use is their SPAM filtering. Please note this is not because I have any negative information and experience. I am a happy SPAMSIEVE user and I have never felt the need to move away from them. Also, I have a Mac that is running mail.app 24/7. Spamsieve provides instructions for configuring so that you can control its operation remotely via your iPhone. Spamsieve at least initially was well ahead of iCloud, Google or FastMail. So I used Spamsieve and never felt the need to investigate other alternatives. Also, I could use the same solution on all three platforms rather than three platform specific approaches.
In my history with FastMail they were bought at one point by Opera. It didn’t appear to change their focus. Eventually, the FastMail staff bought the company back from Opera. Both transitions going to Opera and spinning out of Opera were without any drama which was apparent to the users of the service.
I would recommend you check out the experiences of others at emaildiscussions.com. There is a FastMail testimonial thread in the FastMail sub-forum there.