Gmail seems to have a flaw insofar as it acts as though tags apply only to the first email in a thread rather than to the whole thread (or to all emails in a thread).
This means that if I send an email, tag the thread "to-do" and then get a reply in the same thread, searching for in:inbox label:to-do won't show the thread in the results, despite the fact that the thread is in my inbox and is tagged "to-do".
Searching for in:sent label:to-do will show the thread, but if I'm only interested in finding threads that are in my inbox (rather than archived threads, or messages that I've sent but which haven't yet received replies), then this is no help.
So, is it possible to search by thread rather than by email in Gmail?
Best Answer
Tags are meant for individual messages not threads. The reply will not have the label on it because you have not labelled the message as such. You will want to apply a filter to grab any new messages.
Turn off conservation view and you will be able to search by individual message.
Example filters that could work for your thread
With conversation view off, you should see both the sent message and received message using
label:to-doand only the received items withlabel:to-do in:inbox