ctml Slack is currently being worked on already for the same client. If I'm not mistaken they favor open sourcing that too! They, however, prefer it to be build against the new beta API, you get request access here: https://slack.com/platform-beta
As for alternative mail adapters; they'd need to be able to receive email. A raw IMAP option might be possible but would either be commissioned or contributed. We gladly assist with either of those.