When you delete an email that's part of a thread, that email moves to the trash. The other emails in the thread stay put.
To delete a single email in a conversation:
- While viewing a conversation, expand the email you want to delete.
- Select More (three dots) at the bottom of the message.
- Select Delete Message.
To delete an entire conversation and move all its emails to the trash:
- Select the checkbox next to the conversation.
- Select Delete.