Gmail has a nasty bug (or quirk) that I have only just discovered. It explains why I sometimes have trouble searching for specific emails in my archive. To use Gmail effectively you will need to know about this bug, and how to avoid it.
Background
In Gmail, messages are automatically grouped together into a conversation based on the subject line of the emails. When you reply to a message, or receive a new message with the same subject line, it is added to the conversation.
You can apply labels to a conversation to help organise your archive, and to make it easier to search. For example, I label any JetS3t-related emails I receive with the "jets3t" label. Once I have done this, any responses I send or replies I receive can be found by searching for conversations with the "jets3t" label.
Label Granularity – Conversation or Message?
I had assumed that Gmail applies labels at the conversation level. The UI presentation implies that this is the case, because the label names are displayed at the top of the conversation and it is not possible to apply labels to individual messages. However, it seems that this is not the way Gmail works.
Instead of a label being applied to a whole conversation, it looks like the label is only applied to the messages in a conversation at the time you add the label. Any messages that are added to the conversation later on will not inherit this label.
Misleading search results
I have found several examples where label-based searches that seem straight-forward will not return the results you expect. Two particularly nasty cases where your search results may be misleading are when you apply a star to an individual message in a conversation, or if there is an attachment later on in a conversation. Let's look at a real example of the latter case.
I received a JetS3t-related email with a feature request. I applied the "jets3t" label to this conversation and then replied. After a couple of emails back-and-forth, I sent a message with a PDF file attachment to the person who requested the feature.
Now, let's say I want to find all the JetS3t-related conversations where I have sent or received an attachment. The obvious search to perform is:
label:jets3t has:attachment
However, this does not include my email in the results. I know it should be there, so what gives? I try listing all the messages that have the "jets3t" label:
label:jets3t
This search returns a list of conversations that includes the one in which I sent the attachment, along with hundreds of others. I can also find this particular conversation by searching for all of the messages in my archive that have attachments:
has:attachment
So why doesn't the joint search (label:jets3t has:attachment) find
this conversation? Because although the message I sent is inside a
"jets3t" conversation, the message itself is not associated with this
label. The message is only loosely associated with the "jets3t" label
because it happens to belong to a conversation in which other messages
have this label.
To make this particular search perform properly, I forced Gmail to associate every message in the conversation with the "jets3t" label by re-applying the label to the conversation. After doing this, the joint search finally returned my email in its results.
Beware
This Gmail quirk makes it difficult to use label-based searches with confidence. Unless you perform the tedious work-around of manually re-applying your labels every time a new message is added to a conversation, you cannot be sure that searches against a label name and another criteria will return all the results you would expect.
This bug will affect any message-specific search criteria. In addition to the starred message and attachment cases I have mentioned, it will also affect searches against specific recipients or senders, and searches against particular dates. You should also be aware that the date shown next to a conversation in a label view can be misleading, as it shows the date of the last labelled message rather than the date of the last message in the conversation.
I have found a couple of mentions of this issue, one going back a long time, so I can only assume that this bizarre and opaque behaviour is the way it is meant to be?
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