We've used https://www.enchant.com/ for about a decade for email support .. it's great, does exactly what is is supposed to and doesn't add a bunch of 'cruft' on top. Had to check our volume, currently over 40,000 tickets in that time.
For a long time when we didn't have much support volume we just used a google group that anyone doing tickets could access. This has the nice upside of being free.
We've used Teamwork Desk for the past few years and have been decently satisifed with it. It can be a little clunky at times and the help site feature has had some weird downtime, but for the most part it handles our small support workloads fine.
FreeScout is awesome and self hosted. Been using it for over a year and have been super happy.
Protip: use Gmail SMTP for ultimate deliverability. I personally have a local Postfix forwarding to Gmail. Works great because it keeps a queue and retries if there’s a problem. I found this solution more reliable.
They do live chat too but we've never used that.