FastMail's mail infrastructure is largely built on top of Cyrus IMAP (which tracks reasonably closely to RFCs). FastMail and Kolab (along with many others) also

FastMail's mail infrastructure is largely built on top of Cyrus IMAP (which tracks reasonably closely to RFCs). FastMail and Kolab (along with many others) also have numerous developers who contribute to Cyrus IMAP.