Freshdesk has a so-called Sprout-plan, being free and offering all that an individual would reasonably ask for. I have only played with it for a short while but it looked nice and I could easily integrate my own email-account.

I don’t even remember how I got to Freshdesk. Maybe some ads.

Regrettably, short after I discovered the service, my need for it vanished together with a contract. Hard luck!

If ever I get into a situation, where I need to support users again, this service would be my first choice. My first tests were promising, but even more because there is no reasonably efficient open-source solution to do such things:

• OTRS is great, but complicated to setup and maintain. This is pretty much ok for a large number of supporters, but for a single individual, like a freelancer, this is simply overkill.
• PHP Ticket is simply to simple. I simply mean, it cannot even handle simple attachments.
• …

Looking around on the Internet, these days there are tons of user support systems a.k.a. ticket-systems. Any of them might do. You may want to check out a few test-reports about them. Last time I looked, Freshdesk did not perform too bad. Make your own mind, but … Don’t even try to handle user-support with just a mail-account not supported by a good ticket-system. If you do that you are bound to lose control. You have been warned!