If email is your main channel, I'd start with Help Scout or Front. Freshdesk works if you want proper ticketing rules and SLAs. The main thing I've learned to avoid is wiring your app straight to a shared Gmail inbox and calling it done. The pain points show up later: threading replies, reopening solved tickets, CC handling, ownership, and knowing which customers are still waiting.
Full disclosure: I'm at ClearFeed. It works well if you're open to handling tickets inside Slack instead of email. Your app can create native tickets via API with a title, description, and requester email. email, web chat, and portal tickets can feed into a Slack triage channel. From there you reply, assign, change status, mark solved, add private notes -- everything syncs back to the requester. There's a dashboard too, but daily work stays in Slack.
If you want "message lands in email, I reply from inbox, done," go email-first. If your team lives in Slack and you'd rather not have another dashboard tab become your real workplace, ClearFeed is worth a look
