Can customers/clients acknowledge a closed ticket?

You could make either a customer editable field on the ticket that could be a checkbox for completed that the client could check on their web portal.  If the ticket was closed, a user updating this field would actually cause the ticket to go back into some kind of open state.  You could potentially use an action rule to set the ticket to closed if the client does update the ticket and that field matching how you configure it is in the completed state.

The other option is to send out the surveys on the tickets.

-j_a_catlin

Loop1 Systems: SolarWinds Training and Professional Services

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this a built in feature, once the ticket status has been change to Completed an email is sent for confirmation to the client, you can advance that to Closed if the do agree or reopen ticket if they disagree..

See in ticket settings - Status Types - Under "Completed" status enable - Prompt Client to Confirm Ticket Resolution

Advanced to Close after a period of days if they fail to reply by setting - Automatically Close Ticket After  XX Days