IMAP

Server imap.fastmail.com
Port 993
SSL/TLS Encryption Enabled, but not STARTTLS
Username Your full email address
Password Your app-specific password
Root folder/IMAP path prefix (leave blank)
Folder separator / (forward slash)

Do not enable secure password authentication (SPA). It is not needed when you use SSL/TLS encryption and it will not work.

POP

Server pop.fastmail.com
Port 995
SSL/TLS Encryption Enabled, but not STARTTLS
Username Your full email address
Password Your app-specific password

 

SMTP

Server smtp.fastmail.com
Port 465
SSL/TLS Encryption Enabled, but not STARTTLS
Authentication PLAIN
Username Your full email address
Password Your app-specific password

If your client supports STARTTLS only, you can use port 587 with STARTTLS enabled.