If you are using Fastmail and you want to archive your emails to Mail Archiver then you need to add an application specific password in Fastmail.

Super important

According to their docs at https://www.fastmail.help/hc/en-us/articles/360058752854 IMAP access is only possible if you are at laest on the Standard plan.

Users on a Basic plan will not be able to use Fastmail on third-party mail clients, or create app passwords. IMAP, SMTP, CalDAV, and CardDAV are available to Standard and Professional users only. If you have a Basic plan, you can use Fastmail on a desktop web browser or the Fastmail app on any mobile devices, such as a phone or tablet.

Create an application specific password in Fastmail

Go to Settings -> Password &Security. You have to type in your password again:

If you haven't done so already verify your account with the usual combination of mobile phone and a code:Account verification

After clicking on the "New App Password" button you can select a name for the password and the access type:

Select the second entry "Mail (IMAP/POP/SMTP)". Now you get the app specific password:App specific password in Fastmail

As with every app specific password you need to copy the password somewhere else because after clicking on the "Done" button the password won't be accessible anymore.

Setting up Fastmail in Mail Archiver

This was the hard part. Now it's time for Mail Archiver. Go to the Setup and add a new account. Then select the account type "Other" and click on the "Add" button. 

Enter your IMAP info. The username is username@fastmail.com. The IMAP server is imap.fastmail.com. And the password is the app specific password you created above. After clicking on the "Check" button the green checkmark appears:Fastmail account in Mail Archiver

Now it's possible to add the account to a plan and select mailboxes:

Fastmail account added to plan in Mail Archiver

That's it!