The code is open, you can self-host the application if you prefer not to use their cloud solution[2], it supports PGP email encryption/decryption/signing, and it has an endless number of integrations; you can basically build a support portal with everything your support team needs.
It is a little hard to get started as there is so much to do with it, and so many concepts to learn, but once you set it up the way you want, it is perfect.
If you want something easier, but still self-hosted, Full Help[3] is what I previously used. The developer is super responsive, takes to criticism very well, and engage with you when you have a feature request. Took him less than a month to have PGP encryption support added, from conversation to production.
It is relatively new and has not been updated since October, though, so keep that in mind.
If you want simple and do not want to self-host at all, then Help Scout[4] is your best bet. Their support leaves a lot to be desired, but their product makes up for it.
[1] https://cerb.ai/
[2] I self-host it, personally, but their cloud offering is pretty good.
Plus, if something goes down, we have no one to blame but ourselves, which I see as a nice guarantee for customers as well — there is no lying or blaming the other guy.
[1] That is being generous. We have a few customers in a closed beta, but need to wait until the European borders are open before launching.
[2] Except for our XMPP, email, and EteSync servers, we self-host everything else. These will eventually be self-hosted as well.