this sounds like a helpdesk/shared inbox problem, not a bug tracker problem. mantis isn't bad for bug tracking, but for after-sales email work it feels like the wrong shape.

the big win here probably is how you get out of the whatsapp-group chaos and into one place where every incoming mail becomes a ticket, gets an owner, has internal notes, and stays in one thread when the customer replies. once that happens, stuff stops disappearing into "somebody saw it but nobody owns it."

if you need open source, i'd look at osTicket or Zammad before Mantis. both are much closer to what you're describing: email in, ticket created, assign it, track status, keep the conversation together, add forms if you want more structure. and since your boss already thinks the current mess is "good enough," i'd bias toward the simplest thing your team will actually adopt instead of something feature-heavy that turns into another side project.

i'd also make one process rule with it: after-sale@ becomes the intake point, whatsapp is only for exceptions or urgent escalations. otherwise the tool won't fix much, because the real problem stays in the group chat. :)