Sometimes people see an email 6 months old and just delete it because it's "too old" or refuse to reply because "the customer will think we're a joke taking this long to do something!" This is a great way to lose business, no matter how long it takes to do some tasks, it's always better to get it done. Of course, there are tasks that appear to be irrelevant with the passage of time, so reply and state that you don’t believe it is needed now.
Example: A product suggestion might take 12 months to get around to, but when it's done the customer should be informed and get a link to the new version - it's possible that their need still exists, and they'll know that their ideas and suggestions are not ignored.

- Figure: Bad example - Hitting the “Delete” button without replying
Hi Bob,
This is an old one. I tried to call you but you were not available. Please take a look and let me know if you want it actioned
Figure: Good example - at least asking the question
Hi Bob,
Cleaning my inbox...
Done - a few months ago
Figure: Good example - informing a task was already done
Hi Bob,
Cleaning my inbox...
This still looks relevant so I will start this old task today or tomorrow. Let me know if you don’t want that.
Figure: Good example - giving the option to do it or not
Hi Bob,
Cleaning my inbox... I assume this is too late now. Sorry I missed this before.
Figure: Good example - informing a task is not relevant anymore
Reply to emails regardless of how long it takes to respond. It shows you value the sender's feedback.