This is very common question that i get on community or on other forums on how to identify if an email has been replied or worked upon by a user.

Well the easiest approach to tackle this is that there is a field on emails as parentactivityid  which can be used and put on the views, this field basically will contain the email on which you have replied.

So if this field contains data, it means someone has replied on this email.

for example , see below :

1. My first email doesn’t contain anything in the parentactivityid because its a fresh sent email.
2. My second is actually the email which i have sent above – it has the parent activity id as the first email.
3. My third email is a reply on the previous email received, hence it has that in the parentactivity id column.
4. My last email is the received email which has the parentactivity id as the email which i sent previously.

Hence, the logic should be like.

1.)

Email direction = incoming

parentactivityid = contains email

Action = you have to reply to this email.

2.)

Email direction = outgoing

parentactivityid = contains email 

Action = it means it has been replied.

3.)

Email direction = incoming

parentactivityid = doesn't not contains emails

Action = you have to work on it

4.)

Email direction = outbound 

parentactivityid = doesn't not contain email

Action = its a fresh email.

I hope it makes sense.

Cheers!