Ask HN: What's the best email-based customer support system

We use a combination of Slack and email to track customer issues for our small software company, but it's not working well.

We're looking for something (which must be quite common) which would allow us to track issues in conjunction with the original emails and with Slack discussions. It'd be OK to replace the Slack discussions, but not the email threads. And we don't use gmail, so solutions extending gmail wouldn't help.

Surely, this must be a common need here on HN. Anyone have any recommendations?