Rather than sending a new email to someone you have previously been in touch with by responding to a two month old email from the person in question which has been left in your inbox, create an entirely new email or at least change the subject line to describe what your new email is about (instead of ”Re: [Whatever your old email was about]”).

Remember that in a thread of correspondence one should stick to the point and only discuss one topic at a time. Both you and your recipients will thank your thoughtfulness considering these two points when doing a search for a particular email or topic you discussed.

If you want to, do this

Today, if you find yourself starting a new conversation by replying to an old email from your recipient - instead, write a completely new email to the person in question. 

Add the address to your address book and you will find it more easily the next time you want to send an email.