After searching the forums here, it appears that any number of folks are having difficulty getting the default Mail app to work. No matter what I have tried, I, too, cannot configure the default Mail app to connect to Fastmail.com. It fails when testing the imap connection.
All updates are current on the ElementaryOS device through apt-get.
I was able to install Thunderbird, and it works just fine. I think I would just prefer the interface of the the "native" mail app on eOS.
To be clear, I am aware of the fastmail requirement for using the special device password issued by fastmail and which is required for both Mail a Thunderbird to connect to Fastmail. (if you're not familiar, one has to go to their account on the Fastmail site in a web browser to generate a special password to allow that device to connect to the Fastmail service from an application other than a web browser.)
(why don't I just read my mail in a web browser? I'm old, and I still refuse to use a hammer instead of a screwdriver)