Supporting ManageSieve is a nice touch. Most sieve services only allow managing sieve through a web UI.
I use Fastmail and like that they contribute to open source mail servers, and do standards work (JMAP).
Does PurelyMail contribute to open source?
Some of the libraries I wrote are open sourced and on my Github account, e.g. the web framework: https://github.com/ScottPeterJohnson/shade
I hope you don't find the pain of diverging from the mainline to be too great. We kind of cheated there with Fastmail and Cyrus IMAP by merging all our changes back to the mainline, since there wasn't much other development happening.
(We're also working on JMAP for calendars and for contacts over in the IETF working groups - hoping to publish Calendars by the end of this year)