Here's some info from FastMail blog:
If you are updating a 3rd party client to use an app password, please also switch to the new server names, to save having to update it again in the future.
The new server names are:
IMAP: imap.fastmail.com (port 993, no prefix)
SMTP: smtp.fastmail.com (port 465)
POP: pop.fastmail.com (port 995)
CardDAV:
https://carddav.fastmail.com
CalDAV:
https://caldav.fastmail.com
WebDAV:
https://webdav.fastmail.com
FTP: ftp.fastmail.com (port 21)
There are two important changes compared to the messagingengine.com servers:
You must use an app password. To log in with the new server names you must generate an app password for each app you wish to use; you cannot use your normal FastMail password.
The IMAP server at imap.fastmail.com is set up to use alt-namespace and unix path separators. In plain English, this means you should get better compatibility with email clients (no more top-level folders appearing as sub folders of your Inbox!). There's no need to set an Inbox Path Prefix any more.
As with the *.messagingengine.com server names, all of these protocols MUST be used with an encrypted (TLS/SSL) connection.